New module 'chdir'.

New module 'chdir'.

* modules/chdir: New file.
* lib/unistd.in.h: Include <io.h>, <direct.h> also for chdir.
(chdir): New declaration.
* m4/unistd_h.m4 (gl_UNISTD_H): Test whether chdir is declared.
(gl_UNISTD_H_DEFAULTS): Initialize GNULIB_CHDIR.
* modules/unistd (Makefile.am): Substitute GNULIB_CHDIR.
* tests/test-unistd-c++.cc: Check signature of chdir.
* doc/posix-functions/chdir.texi: Mention problem on native Windows.
* modules/chdir-long (Depends-on): Add chdir.
* modules/fchdir (Depends-on): Likewise.
* modules/rename (Depends-on): Likewise.
* modules/savewd (Depends-on): Likewise.