Win32: fix environment memory leaks

Win32: fix environment memory leaks

All functions that modify the environment have memory leaks.

Disable gitunsetenv in the Makefile and use env_setenv (via mingw_putenv)
instead (this frees removed environment entries).

Move xstrdup from env_setenv to make_augmented_environ, so that
mingw_putenv no longer copies the environment entries (according to POSIX
[1], "the string [...] shall become part of the environment"). This also
fixes the memory leak in gitsetenv, which expects a POSIX compliant putenv.

[1] http://pubs.opengroup.org/onlinepubs/009695399/functions/putenv.html

Note: This patch depends on taking control of char **environ and having
our own mingw_putenv (both introduced in "Win32: Unicode environment
(incoming)").

Signed-off-by: Karsten Blees <blees@dcon.de>