Win32: simplify internal mingw_spawn* APIs

Win32: simplify internal mingw_spawn* APIs

The only public spawn function that needs to tweak the environment is
mingw_spawnvpe (called from start_command). Nevertheless, all internal
spawn* functions take an env parameter and needlessly pass the global
char **environ around. Remove the env parameter where it's not needed.

This removes the internal mingw_execve abstraction, which is no longer
needed.

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Stepan Kasal <kasal@ucw.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>