Move the rc framework out of sbin/init into libexec/rc.

Move the rc framework out of sbin/init into libexec/rc.

The reasons for this are forward looking to pkgbase:
 * /sbin/init is a special binary; try not to replace it with
   every package update because an rc script was touched.
   (a follow-up commit will make init its own package)
 * having rc in its own place will allow more easy replacement
   of the rc framework with alternatives, such as openrc.

Discussed with:		brd (during BSDCam), kmoore
Requested by:		cem, bz
PR:			231522
Approved by:		re (gjb)