Fix ddb rc script

r288291 added a call to limits(1), which isn't available before partitions
are mounted. This broke the ddb rc script, which does not provide its own
start_cmd.

Alleviate the situation here by providing a start_cmd. We still have other
problems with diskless setups that need to be considered, but this is a
start.

PR:		206291
Submitted by:	cy
Discussed with:	rgrimes
MFC after:	3 days

etc/rc.d/ddb

@@ -15,6 +15,7 @@ desc="DDB kernel debugger"
 rcvar="ddb_enable"
 command="/sbin/${name}"
 start_precmd="ddb_prestart"
+start_cmd="ddb_start"
 stop_cmd=":"
 
 ddb_prestart()
@@ -25,6 +26,11 @@ ddb_prestart()
 	fi
 }
 
+ddb_start()
+{
+	${command} ${command_args}
+}
+
 load_rc_config $name
 
 required_files="${ddb_config}"