stat: Fix time_t values and other problems on native Windows platforms.
* doc/posix-functions/stat.texi: Mention the problem with the Microsoft
implementations of stat().
* lib/stat.c: Include filename.h instead of dosname.h. Include
malloca.h, stat-w32.h.
(is_unc_root): New function.
(rpl_stat): New implementation for native Windows. Remove
REPLACE_FUNC_STAT_DIR code.
* m4/stat.m4 (gl_FUNC_STAT): On native Windows, set REPLACE_STAT always.
Don't define REPLACE_FUNC_STAT_DIR.
(gl_PREREQ_STAT): Require gl_HEADER_SYS_STAT_H.
* modules/stat (Files): Add lib/stat-w32.h, lib/stat-w32.c.
(Depends-on): Remove dosname. Add filename, malloca.
(configure.ac): Also compile lib/stat-w32.c.