Allow changing of bogons update frequency

As concerns have been raised with the update duration of the fullbogons lists in the forum [1], I have added an option to increase the update frequency. It does not change the default update frequency such as not to increase the required bandwidth too much.

[1]: http://forum.pfsense.org/index.php/topic,57286.0.html

Edit:
- Added support for custom bogon urls in rc.update_bogons script.
- Added support for compressed bogon files in rc.update_bogons script

To change the default URLs the following variables can be set in "/var/etc/bogon_custom"

    v4url="http://files.pfsense.org/lists/fullbogons-ipv4.txt"
    v6url="http://files.pfsense.org/lists/fullbogons-ipv6.txt"
    v4urlcksum=${v4url}.md5
    v6urlcksum=${v6url}.md5