Use find -exec with '+' for grep-find if supported.

Use find -exec with '+' for grep-find if supported.

* lisp/progmodes/grep.el (grep-find-use-xargs): Doc fix.
(grep-compute-defaults): Check for `-exec COMMAND +' support.
Set grep-find-use-xargs, grep-find-command, and grep-find-template
accordingly.  Don't add the null-device if not needed.