Set up cscope when it is available.
Actual fixes for C mode.

Fixes bugs from bc6f6fc ("Fixup for C modes in Emacs24").
Use the silver searcher instead of ack.

It's much faster.
Add shortcut keybindings for some cscope defuns.
Nuke some old calls to cscope:hook.
custom-set-variables for cscope.