devmatch: Address some rc nits

- devmatch_enable in rc.conf(5) was not gating the start of devmatch

- Use quietstart in devd/devmatch to suppress dozens of 'Cannot start'
  messages and other spurious messages from rc.subr(8) that aren't
  necessarily helpful.

Discussed with:	imp

etc/devd/devmatch.conf

@@ -9,7 +9,7 @@
 #
 # Generic NOMATCH event
 nomatch 100 {
-	action "service devmatch start '?$_'";
+	action "service devmatch quietstart '?$_'";
 };
 
 # Add the following to devd.conf to prevent this from running:

etc/rc.d/devmatch

@@ -33,6 +33,7 @@
 
 name="devmatch"
 desc="Use devmatch(8) to load kernel modules"
+rcvar="${name}_enable"
 
 start_cmd="${name}_start"
 stop_cmd=':'