Allow toggling Grep command abbreviation, and rename related symbols

Allow toggling Grep command abbreviation, and rename related symbols

* lisp/progmodes/grep.el (grep-find-hide): Rename 'grep-find-hide'
to 'grep-find-abbreviate'.
(grep-find-hide-properties): Rename to
'grep-find-abbreviate-properties'.
(grep-mode-font-lock-keywords): Even when 'grep-find-abbreviate'
is nil, fontify the verbose command options with property
'abbreviated-command', so that the toggling command can later find
these parts without refontifying.
(grep-find-show): Rename to 'grep-find-toggle-abbreviation',
simplify the code, and permit toggling the hidden command options.
(grep-mode-map): Add a menu-bar item and new separator for
'grep-find-toggle-abbreviation', and fix the incumbent separators
in the "Grep" menu-bar, of which only one was showing before this
commit, even though two were specified.
* etc/NEWS (grep):
* doc/emacs/building.texi (Grep Searching): Document these
changes, and mention 'grep-find-toggle-abbreviation'.