rc.subr(8): 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
MFC after: 2 weeks
Reported by: Dmitry S. Lukhtionov
Tested by: Dmitry S. Lukhtionov