Since m_option.h and options.h are extremely often included, a lot of
files have to be changed.
Moving path.c/h to options/ is a bit questionable, but since this is
mainly about access to config files (which are also handled in
options/), it's probably ok.
Remove the ifdef hell from mp_find_user_config_file(). Move the win32
specific code (for MinGW and Cygwin) to path-win.c. The behavior should
be about the same, but I can't be sure due to lack of testing and
because the old path.c code was hard to follow. (I expect those who care
about windows will fix things, should issues pop up - sorry.)
One difference is that the new code will always force MPV_HOME. It looks
like the old code preferred the mpv config dir in the exe dir if it
exists.
Also, make sure MP_PATH_MAX has enough space, even if the equivalent
wchar_t string is not 0-terminated with PATH_MAX (because apparently the
winapi doesn't require this). (Actually, maybe we should just kill all
uses of PATH_MAX/MP_PATH_MAX.)
