Remove support for IRIX

Remove support for IRIX

The IRIX port wasn’t really working anyway, and the code was
getting in the way of other changes (e.g., getaddrinfo fixes).
IRIX’s supplier dropped support for IRIX in 2013.
* admin/CPP-DEFINES:
* configure.ac (opsys):
* doc/lispref/os.texi (System Environment):
* etc/MACHINES, etc/PROBLEMS:
* lisp/find-dired.el (find-grep-options):
* lisp/lpr.el (lpr-lp-system):
* lisp/ls-lisp.el (ls-lisp-emulation):
* lisp/mail/rmail.el (rmail-spool-directory):
* lisp/net/net-utils.el (ping-program-options):
* lisp/progmodes/gud.el (gud-irix-p, gud-dbx-use-stopformat-p):
* lisp/progmodes/ps-mode.el (ps-mode-print-function):
* src/conf_post.h [IRIX6_5]:
* src/emacs.c (Vsystem_type):
* src/filelock.c (get_boot_time_1):
* src/process.c (process_send_signal):
* src/unexelf.c (unexec):
Omit IRIX-specific code and/or documentation.
* configure.ac (NON_GCC_TEST_OPTIONS, IRIX6_5, PREFER_VSUSP):
Remove.  All uses removed.
* etc/NEWS: Document the change.