Add `r'/`l' grep command history commands

Add `r'/`l' grep command history commands

* doc/emacs/building.texi (Grep Searching): Mention the
`r'/`l' commands.

* lisp/progmodes/grep.el (grep-forward-history): New command.
(grep-backward-history): Ditto.
(grep--save-buffers): New function.
(grep): Use it to record the history.
(grep--command-history, grep--history-inhibit)
(grep--history-place): New internal variables for the grep
history (bug#22627).