Check for tmp captiveportal dir before making it

In forum: https://forum.pfsense.org/index.php/topic,72483.0.html
Warning: mkdir(): File exists in /etc/inc/system.inc on line 878
Another little change to get rid of a warning message - could also go into 2.1.1. Seems there are never-ending little things to find for 2.1.1.
Not sure if you would rather call safe_mkdir here? and in other places in this file that have the "if(!is_dir) then mkdir" sequence.