Implement OS-based firewalling to protect iSymphony module

Official iSymphony security advisory says that OS-based firewall protection of ports 50001-50003 should be implemented.
Default installation of FreePBX distro does not implement this protection, leaving vulnerable ports open.
Please see:
http://docs.getisymphony.com/display/SEC/Security+Advisory+2012-09-13

Fix is to add following rules to iptables config:

iptables -A INPUT -p tcp --dport 50001 -j DROP
iptables -A INPUT -p tcp --dport 50002 -j DROP
iptables -A INPUT -p tcp --dport 50003 -j DROP