Declare many etags command obsolete; update the goto menu

* lisp/menu-bar.el (menu-bar-goto-menu): Replace all but one etags item
with xref ones.

* lisp/progmodes/etags.el (find-tag-other-window)
(find-tag-other-frame, find-tag-regexp, tags-loop-continue)
(tags-apropos): Declare obsolete.
* lisp/vc/vc-dir.el (vc-dir-display-file): New command.

(vc-dir-mode-map): Bind it to "\C-o".
(vc-dir-menu-map): Add it to menu.

Fixes: debbugs:19450
More fixes to support multi-line search in comint

* lisp/comint.el (comint-history-isearch-message): Use field-beginning
instead of comint-line-beginning-position - that's more fixes for
http://lists.gnu.org/archive/html/emacs-devel/2014-12/msg00305.html
(comint-history-isearch-message): Fix args of isearch-message-prefix.
Small fixes in etc/grep.txt