Remove references to OS/2 in code, doc, and comments

Remove references to OS/2 in code, doc, and comments

* lisp/gnus/nnheader.el (nnheader-read-timeout)
(nnheader-file-name-translation-alist): Remove OS/2 case, and simplify.
* lisp/emulation/viper-util.el (viper-color-defined-p):
* lisp/net/pop3.el (pop3-read-timeout):
* lisp/net/imap.el (imap-read-timeout):
* lisp/url/url-privacy.el (url-setup-privacy-info): Remove OS/2 case.
* lisp/emulation/viper-ex.el (viper-glob-function):
* lisp/vc/ediff-util.el (ediff-submit-report): Doc fix.
* lisp/cus-edit.el (custom-display): Remove "pm" (OS/2).
* doc/emacs/msdos-xtra.texi (MS-DOS):
* doc/misc/gnus.texi (Various Various):
* doc/misc/viper.texi (Rudimentary Changes): Remove mentions of OS/2.
; * lisp/url/url.el, lisp/url/url-vars.el:
; * lisp/progmodes/cperl-mode.el: Remove old comments.