Convert more defvars to defcustoms.

Convert more defvars to defcustoms.

* dos-w32.el (file-name-buffer-file-type-alist)
(direct-print-region-use-command-dot-com):
* ffap.el (ffap-menu-regexp):
* follow.el (follow-debug):
* forms.el (forms--debug):
* iswitchb.el (iswitchb-all-frames):
* ido.el (ido-all-frames):
* mail/feedmail.el (feedmail-mail-send-hook)
(feedmail-mail-send-hook-queued):
* mail/footnote.el (footnote-signature-separator):
* mail/mailabbrev.el (mail-alias-separator-string)
(mail-abbrev-mode-regexp):
* mail/rmail.el (rmail-speedbar-match-folder-regexp):
* progmodes/idlwave.el (idlwave-libinfo-file)
(idlwave-default-completion-case-is-down)
(idlwave-library-routines): Convert defvars to defcustoms.

* mail/rmail.el (rmail-decode-mime-charset):
* progmodes/idlw-shell.el (idlwave-shell-print-expression-function)
(idlwave-shell-fix-inserted-breaks)
(idlwave-shell-activate-alt-keybindings)
(idlwave-shell-use-breakpoint-glyph):
* facemenu.el (facemenu-unlisted-faces): Delete obsolete vars.

* doc/lispref/os.texi (Timers): Use defopt for timer-max-repeats.