maint: move stat and lstat from posix.unistd to posix.sys.std.

* ext/posix/unistd.c (filetype, Fstat, Sstat, Plstat, Pstat):
Move from here...
* ext/posix/sys/stat.c (filetype, Fstat, Sstat, Plstat, Pstat):
...to here.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>