Revert r263754, re-adding support for hw.bus.devctl_disable.  Breaking
old devd's and thus hosts that get IP addresses from DHCP was too much
of a POLA violation.

The sysctl may be removed again after r263758 has been merged to at
least stable/9 and stable/10, and releases have been cut from those
branches.

Discussed with:	mjg
Reported by:	theraven, rwatson
Remove note about hw.bus.devctl_disable removal, restored
in r265060.

Sponsored by:	The FreeBSD Foundation