MFC: r249410 (partial)

Remove ctl(4) from GENERIC.

With ctl(4) being built as a module and automatically loaded by ctladm(8),
this makes CTL work out of the box.

Note that unlike the original r249410, this commit does not remove the
kern.cam.ctl.disable tunable for POLA reasons.

PR:		174671, 175694, 179112
Reviewed by:	ken (original version)
Sponsored by:	FreeBSD Foundation (original version)
Approved by:	re (hrs)