Add webcam thread to avoid COM-related crash

Webcam code was sometimes causing crashes in Meterpreter when attempting
to stop the camera after a frame had been captured. This appeared to be
because the thread that started the capture was not the same thread that
ended it.

CoInitialize() and CoUninitialize() need to be called on the same thread
and objects created on one thread need to be released on the same thread.
This change results in a new worker thread that is used for the lifetime
of the capture, and the callers have to interact with this thread via
basic threading events.

This is currently "proof of concept" code, rather than production-ready
code. The goal is to see if it solves the issue on the known targets
before tidying it up and locking it down for release.
Command refactor for clean shutdown + inline calls

This work contains a bunch of changes around command dispatching. The
goals for this bit of work were to:

* Provide the ability for commands to be executed on the same thread as
  the server rather than always creating new threads and executing them on
  those threads.
* Have the means for _special_ commands, such as `exit` and `migrate` to
  shut down the server cleanly without having to rely on signalling across
  threads or by doing brutal thread termination via shared global handles.
  This should not only fix the dirty shutdown problem on Windows which
  leaves tasks dangling (or based on the prior attempt at fixing, crashing
  stuff as well), it should also help clean up the shutdown process in
  POSIX.

These changes hit a very important part of Meterpreter and so should be
reviewed with intense scrutnity. I expect this PR to garner a log of
critique and most likely a number of changes before being included in the
main line.

The `PacketDispatcher` was modified to include a new function pointer
called an `inline_handler`. This new member indicates that there's a
handler which should be invoked inline. While this sits alongside the
existing `handler`, they are actually mutually exclusive. If an
`inline_handler` is specified then the `handler` is ignored and it is
assumed that the command is intended to be handled inline. The signature
of the inline handler is different to the standard handler, and this is
why a new function pointer was added rather than a simple flag. Addition of
this parameter meant that the basic command structure changed, and that
obviously affects all of the extensions and their respective commands.
This changeset therefore updates each of those command declarations so
that they use the new macros that hide this detail.

Other things to be mindful of:

* This version of the code reads the command's `method` prior to invoking
  any other function, and after that the command itself is passed around to
  the threaded or non-threaded routes for invocation. An extra thread
  parameter was included as as result, and an overload for the
  `thread_create` function was added which supported this new parameter.
  This was named `thread_create3` because
  `thread_create_with_another_paramter` sounded a bit crap.
* The migration code, which originally had a `thread_kill` and an event
  wait once the new meterpreter session had been created, has been modified
  to not do any waiting at all. Instead it finishes execution as fast as
  possible and returns control to the server which should respond by
  shutting down in a clean way.
* Originally the code always attempted to call a command handler in the
  base command list and then, if found, would also call an "overload" in
  the extension commands list. From the investigation that I did, it
  appears that the overloaded methods did nothing in the base (they'd
  early out during invocation). As a result, the new way of doing things
  acts like a 'true' overload in that the extension commands are searched
  first, and if one is found this is the command that is executed. Any
  base commands with the same method name will not get executed. In the
  case where there is no extension command found, the base command list is
  then queried. If a command is found that command is instead invoked.
* The POSIX version still compiles cleanly, but I've never been able to
  build a version that runs on my machines. I'm not sure if there's a
  trick to getting POSIX builds to run, and if there is I don't know it.
  Whoever scrutinises this build should make sure that the POSIX version
  that they build can still run and (hopefully) exit cleanly.

I've added lots of documentation, but there's always room for improvement.

Hopefully this will fix the `*_tcp` side of Redmine 8438.

Bring on the feedback!
PR tidy based on feedback

* Updated `thread_create` so that it has 3 parameters, and removed
  `thread_create3`.
* Updated all calls to `thread_create` and added the extra parameter of
  `NULL`.
* Fixed comment typo.
* Removed assignment where value is not used.
* Checked for `NULL` prior to setting the result.
* Undefined `DEBUGTRACE`.
Fix invalid thread_create calls on posix

I'm such a noob. My grep-fu was weak with this one.
Fix small issue with make

Make now correctly loads the environment automatically if it can find it.
Interactive channel refactor

The goals of this work are:

* To fix issue where backgrounding and re-interacting with channels wasn't
  working.
* To fix issue where closing of meterpreter was not closing off background
  prcoesses (such as cmd.exe).

The two things preventing this stuff from working were:

* When interactive channels are backgrounded their handles were destroyed
  along with the context that wraps them up. Making them interactive again
  had no impact because the handle and context were invalid. If anything,
  this made meterpreter unstable. Sometimes the session would die when
  attempting to interact with the channel again.
* When closing channels, there was no way of terminating the process that
  sat behind the scenes because no reference to the process was retained.
  Channels would close and handles would close, but no process termination
  was done.

To fix these problems:

* The interactive thread no longer terminates when backgrounded. Instead
  its put in a suspended state where it's waiting a signal from a resume
  handle that's associated with the channel's context. This means that the
  destruction of the context doesn't happen at all until the termination
  of the channel, which is exactly when it should happen anyway.
* Process handles are stored alongside the input/output handles so that
  when the time comes, the process can be terminated if required. This
  means that when the channels are closed, the code has a reference to the
  associated process which can be terminated. This is only done for
  interactive processes, non-interactive processes do not have this
  problem because meterpreter doesn't have to keep track of them.
Fix up POSIX to match channel changes

POSIX was out of whack with Windows as a result of the changes made
around channels. The schedular in posix was very different, and this
commit brings it into line.

Other than the obvious issues, a non-obvious issue with the changes
was that the channel was being freed up on close prior to the thread
terminating. This doesn't appear to be an issue on Windows, but was
causing crashes on close in POSIX.

The changes go quite deep. This changeset requires a lot of testing.
Final Windows changes to match POSIX requirements

This changeset brings windows into line with the last set of POSIX
changes. With this changeset we are now in a position where both POSIX and
Windows are able to create and open interactive channels, put them in the
background, and terminate them without crashing, hanging or leaving
processes running behind the scenes.
Remove debug flag
Re-add closing of handles on destroy
Fix ipconfig command to show IPv6

This commit also contains fixes for proper extraction of subnet masks
based on operating system.
Merge branch 'upstream/master' into command_refactor

Conflicts:
	make.bat
Merge branch 'master' into ipconfig_ipv6
Minor README update warning against VS2012.
Merge branch 'master' into webcam_crash_fix
Clean, tidy and add documentation

This commit tidies up the webcam code, adds documentation and adds a
couple of small clean-ups and optimisations around resource usage.

`audio.h` is not included in `webcam.h` any more as it's not needed at all
by that code, however it has been added to `precomp.h` so that `stdapi.c`
can use and see it along with the other files.
Remove unused forward declarations
Remove use of BREAK_ON_ERROR from posix code

POSIX no likey BREAK_ON_ERROR, it fails at runtime. Replaced with portable code.
Docs for list, xor and remote
Land #48, containing #39, #42, and #43

This lands #39, #42, and #43. #34 still needs work.