New module 'dup'.

* lib/unistd.in.h (dup): Declare only if the 'dup' module is in use.
Allow replacement.
* lib/dup.c: New file.
* lib/fchdir.c (rpl_dup): Remove function.
* m4/dup.m4: New file.
* m4/fchdir.m4 (gl_FUNC_FCHDIR): Don't set REPLACE_DUP here.
* m4/unistd_h.m4 (gl_UNISTD_H): Test whether 'dup' is declared.
(gl_UNISTD_H_DEFAULTS): Initialize GNULIB_DUP.
* modules/unistd (Makefile.am): Substitute GNULIB_DUP.
* modules/dup: New file.
* tests/test-unistd-c++.cc: Check the signature of 'dup' only if the
'dup' module is in use.
* modules/fdopendir (Depends-on): Add dup.
* modules/fdutimensat-tests (Depends-on): Likewise.
* modules/fts (Depends-on): Likewise.
* modules/futimens-tests (Depends-on): Likewise.
* modules/posix_spawnp-tests (Depends-on): Likewise.
* modules/unistd-safer-tests (Depends-on): Likewise.
* modules/utimens-tests (Depends-on): Likewise.
* doc/posix-functions/dup.texi: Mention the new module and the problem
on MSVC.