merge patch from http://jira.freeswitch.org/browse/SFSIP-11 .  Please note, when registering to a UA that requires the from header to match the auth username, you must now use the new setting force-fromuser.  See sample configuration file for more details.  Thanks Marcel.

git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@4970 d0543943-73ff-0310-b7d9-9358b9ac24b2