grantpt: Relax requirement regarding invalid file descriptors.

grantpt: Relax requirement regarding invalid file descriptors.

* lib/grantpt.c: Don't include <fcntl.h>.
(grantpt): Don't verify the validity of the file descriptor.
* modules/grantpt (Depends-on): Remove fcntl-h.
* tests/test-grantpt.c (main): Allow grantpt to succeed for invalid
file descriptors.
* doc/posix-functions/grantpt.texi: Document more platforms on which
grantpt succeeds for invalid file descriptors.
Reported by Rich Felker <dalias@aerifal.cx>.