Perform grep-mode's buffer modifications in a process filter (Bug#7952)

Perform grep-mode's buffer modifications in a process filter (Bug#7952)

* progmodes/grep.el (grep-mode-font-lock-keywords): Remove
buffer-changing entries.
(grep-filter): New function.
(grep-mode): Add it to compilation-filter-hook.

* progmodes/compile.el (compilation-filter-hook)
(compilation-filter-start): New defvars.
(compilation-filter): Call compilation-filter-hook prior to
updating the process mark.