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.

lisp/net/net-utils.el

@@ -79,7 +79,7 @@
 ;; On GNU/Linux and Irix, the system's ping program seems to send packets
 ;; indefinitely unless told otherwise
 (defcustom ping-program-options
-  (and (memq system-type '(gnu/linux irix))
+  (and (eq system-type 'gnu/linux)
        (list "-c" "4"))
   "Options for the ping program.
 These options can be used to limit how many ICMP packets are emitted."