Add compilation-minor-mode to minor-mode-alist and minor-mode-map-alist.

Add compilation-minor-mode to minor-mode-alist and minor-mode-map-alist.
(compilation-minor-mode): New defvar.
(compilation-minor-mode-map): New defvar; keymap with the bindings
from compilation-mode-map except SPC and DEL.
(compilation-mode-map): Construct this keymap to inherit from
compilation-minor-mode-map.
(compilation-setup): New function, code broken out of compilation-mode.
(compilation-mode): Call it.
(compilation-minor-mode): New function to toggle
compilation-minor-mode; if setting it, call compilation-setup.