mk: Add some serious documentation and 'make help'

'make help' is implemented in a fairly ridiculous way, using awk
to parse comments out of Makefile.in and displaying them.
mk: Remove the concept of 'snapshot transitions'

This way of doing snapshots hasn't been used since 2011.
mk: Move most of Makefile.in to .mk files

Because the build system treats Makefile.in and the .mk files slightly
differently (.in is copied, .mk are included), this makes the system
more uniform. Fewer build system changes will require a complete
reconfigure.
mk: Add NO_MKFILE_DEPS for turning off rebuild from makefile changes
mk: Improve build system help commands
mk: Move version info to top of main.mk

Just so it's easier to find.
mk: Address review feedback
auto merge of #12274 : brson/rust/mkfiles, r=alexcrichton

I've been working on binary installers and ended up taking this detour, which does a few things:

* It expands the documentation on the build system with new comments in Makefile.in
* It displays some of that documentation via `make help`
* Removes some unused and broken snapshot code
* Adds `NO_MKFILE_DEPS` to convenience makefile hacking
* Moves almost all of Makefile.in to files in `mk/`

The documentation provided by `make help` and its implementation are somewhat quirky.
Add c_str::CString.as_bytes_no_nul()
Use str::from_utf8_lossy() in os::args(), add os::args_as_bytes()

os::args() was using str::raw::from_c_str(), which would assert if the
C-string wasn't valid UTF-8. Switch to using from_utf8_lossy() instead,
and add a separate function os::args_as_bytes() that returns the ~[u8]
byte-vectors instead.
Use str::from_utf8_lossy() for os::env() and friends

Parse the environment by default with from_utf8_lossy. Also provide
byte-vector equivalents (e.g. os::env_as_bytes()).

Unfortunately, setenv() can't have a byte-vector equivalent because of
Windows support, unless we want to define a setenv_bytes() that fails
under Windows for non-UTF8 (or non-UTF16).
Create RefReader and RefWriter adaptor structs

RefReader and RefWriter allow a caller to pass a Reader or Writer
instance by reference to generic functions that are expecting arguments
by value.
Update LimitReader to take the Reader to wrap by value
Remove broken link to old conditions tutorial
auto merge of #12230 : DaGenix/rust/io-decorator-changes, r=sfackler

I created RefReader and RefWriter structs that wrap a mutable reference to a Reader or Writer value. This works exactly like the ByRef struct in the iter module and allows passing a reference to a Reader or Writer to function expecting a Reader or Writer by value with the caller retaining ownership to the original value.

I also modified LimitReader to take the wrapped Reader by value instead of by reference.

@sfackler
auto merge of #12283 : kballard/rust/env-args-bytes, r=erickt

Change `os::args()` and `os::env()` to use `str::from_utf8_lossy()`.
Add new functions `os::args_as_bytes()` and `os::env_as_bytes()` to retrieve the args/env as byte vectors instead.

The existing methods were left returning strings because I expect that the common use-case is to want string handling.

Fixes #7188.
auto merge of #12286 : sfackler/rust/no-conditions, r=alexcrichton