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.
Merge branch 'upstream/master' into command_refactor

Conflicts:
	make.bat
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.
Fix typo
Land #34, command refactor