* progmodes/grep.el (grep-compute-defaults): Simplify how settings

* progmodes/grep.el (grep-compute-defaults): Simplify how settings
are looked up.
(grep-apply-setting): New function.
(grep-highlight-matches, grep-command, grep-template)
(grep-use-null-device, grep-find-command, grep-find-template):
Clarify role of grep-compute-defaults in docstrings.  Use
grep-apply-setting to apply changes (Bug#3343).