unistd-h: Make sure O_CLOEXEC is defined. * modules/unistd-h (Depends-on): Add fcntl-h. * lib/unistd.in.h: Include fcntl.h if inclusion of unistd.h does not define O_CLOEXEC. * doc/posix-headers/unistd.texi: Document the platforms that do not define O_CLOEXEC in unistd.h.