(captiveportal, new) add captiveportal_enable in rc script

src/etc/rc.d/captiveportal

@@ -28,10 +28,13 @@
 . /etc/rc.subr
 
 name=captiveportal
+rcvar=captiveportal_enable
 start_precmd=captiveportal_prestart
 start_cmd="${name}_start"
 stop_cmd="${name}_stop"
 
+[ -z "$captiveportal_enable" ]       && captiveportal_enable="NO"
+
 captiveportal_load_rc_config()
 {
     CPWORKDIR="/var/captiveportal"