(grep-find-use-xargs): Use explicit value `exec'

(grep-find-use-xargs): Use explicit value `exec'
to mean "use find -exec"; nil now unambiguously means auto-detect.
(grep-compute-defaults): Set grep-find-use-xargs to `exec' if not `gnu'.
Use shell-quote-argument to build grep-find-command and grep-find-template.
(rgrep): Use shell-quote-argument to properly quote arguments to find.
Reported by Tom Seddon.