largefile: Improve and document. * m4/largefile.m4 (gl_LARGEFILE): Set WINDOWS_64_BIT_ST_SIZE to 0 if the mingw headers already define 'stat' appropriately. * modules/largefile (Description): Clarify. * doc/largefile.texi: New file. * doc/gnulib.texi: Include it. * doc/posix-headers/sys_types.texi: Update.