ptsname_r: Fix test failures on IRIX, Solaris.

ptsname_r: Fix test failures on IRIX, Solaris.

* m4/ptsname_r.m4 (gl_PREREQ_PTSNAME_R): Test whether isatty sets
errno when it fails. Define ISATTY_FAILS_WITHOUT_SETTING_ERRNO
accordingly.
* lib/ptsname_r.c: Include <fcntl.h>.
(__ptsname_r): When isatty returned false, then on IRIX, Solaris
set errno if fd is invalid.
* tests/test-isatty.c (main): Update comments.