(defined_S_IFMT): New macro.

(defined_S_IFMT): New macro.
Include stat-macros.h.
Include stdlib.h, for abort().
Don't include stdio.h or assert.h; no longer needed.
(same_file_type): Don't assume S_IFMT is defined, as POSIX
does not require this.  Don't assume S_IFCHR and S_IFBLK have
their usual sort of bit pattern.
(fchmod_new): Open with O_NOCTTY for as well, for minor
improvement on hosts where that matters.  Don't bother to assert,
since the caller (in this source file) checks the same thing.
Discard any errno from a close failure, for consistency with other
code.