stat: new module, for mingw bug

stat: new module, for mingw bug

Depending on the current directory, either stat(".",buf) or
stat("./",buf) would fail on mingw.

* modules/stat: New file.
* lib/stat.c: Likewise.
* m4/stat.m4 (gl_FUNC_STAT): Likewise.
* m4/sys_stat_h.m4 (gl_SYS_STAT_H_DEFAULTS): Add witnesses.
* modules/sys_stat (Makefile.am): Use them.
* lib/sys_stat.in.h (stat): Declare replacement.
* lib/openat.c (fstatat): Deal with lstat and stat being function
macros.
* modules/openat (Depends-on): Add inline.
* MODULES.html.sh (systems lacking POSIX:2008): Mention module.
* doc/posix-functions/stat.texi (stat): Likewise.
* modules/stat-tests: New test.
* tests/test-stat.c: Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>