rc.subr(8): MFC: introduce ${name}_offcmd

New variable ${name}_offcmd may be used to supply commands
executed if named service is not enabled. Previously start_precmd
could be used for such a task but now rc.subr(8) does not call it
if a service is not enabled.

Fix devd startup script to use it instead of start_precmd.

PR: 279198
Reported by: Dmitry S. Lukhtionov
Tested by: Dmitry S. Lukhtionov

(cherry picked from commit 32a579e4fc69a65e8901111ad5f65ec56a97dfab)
(cherry picked from commit c2db3a0c7d31116028b38b426a9b139d26cbc7e5)