Update res_phoneprov to default to setting the SERVER variable to the IP

Update res_phoneprov to default to setting the SERVER variable to the IP
the HTTP request for the config came in on and the SERVER_PORT to the
bindport setting in sip.conf.  I've left in the ability to override these
options, because I can't always guess how someone might decide to do something
weird with what is available to them--although needing to is pretty unlikely.

Documentation was updated to reflect preference for not setting serveraddr,
serveriface, or serverport.  Tested on Linux and OS X.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@98988 65c4cc65-6c06-0410-ace0-fbb531ad65f3