(grep): New defcustom group.
(grep-window-height): New defcustom, like compilation-window-height.
(grep-auto-highlight): New defcustom, like compile-auto-highlight.
(grep-scroll-output): New defcustom, like compilation-scroll-output.
(grep-command, grep-use-null-device, grep-find-command)
(grep-tree-files-aliases, grep-tree-ignore-case)
(grep-tree-ignore-CVS-directories): Move to grep custom group.
(grep-setup-hook): New hook variable.
(grep-mode-map): New keymap for grep commands. Add Grep menu.
(grep-use-compilation-buffer): New defcustom.
(grep-last-buffer): New defvar, override compilation-last-buffer.
(grep): Add optional arg HIGHLIGHT-REGEXP. Doc fix.
Call compile-internal with args highlight-regexp and grep-mode-map.
