As a function that appears to only be called on error paths, I don't
think it makes sense for it to return an error, or clobber the global
giterr. Note that no existing callsites actually check the return
code.

In my own application, there are errors where the real error ends
up being hidden, as git_mwindow_file_deregister() clobbers the
global giterr. I'm not sure this error is even relevant?

Up to now, the idea was that the user would do all the operations for
one repository in the same thread. Thus we could have the
memory-mapped window information thread-local and avoid any locking.

This is not practical in a few environments, such as Apple's GCD which
allocates threads arbitrarily or the .NET CLR, where the OS-level
thread can change at any moment.

Make the control structure global and protect it with a mutex so we
don't depend on the thread currently executing the code.

Once a file is registered, there is no way to deregister it, even
after the structure that contains it is no longer needed and has been
freed. This may be the source of #624.

Allow and use the deregister function to remove our file from the
global list.

This converts blob.c, fileops.c, and all of the win32 files.
Also, various minor cleanups throughout the code.  Plus, in
testing the win32 build, I cleaned up a bunch (although not
all) of the warnings with the 64-bit build.

Signed-off-by: schu <schu-github@schulog.org>

See `global.c` for a description of what we're doing.

When libgit2 is built with GIT_THREADS support, the threading system
must be explicitly initialized with `git_threads_init()`.

1. The license header is technically not valid if it doesn't have a
copyright signature.

2. The COPYING file has been updated with the different licenses used in
the project.

3. The full GPLv2 header in each file annoys me.

On the way, store the fd and the size in the mwindow file.

Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>