Add "userfield" parameter to configuration files like "accountcode".

Contact: Richard Cook (richard@sivana.ca)
IRC: freenode - sivana


NOTE — THIS BOUNTY HAS BEEN FULFILLED. Payment has been made.

This patch will *not* make it into CVS HEAD for Asterisk. I have decided to post the code to be included
in the new OpenPBX (Asterisk Fork) — learn more here: http://www.openpbx.org