Save the 'From' header received in a REGISTER message so we can show it

Save the 'From' header received in a REGISTER message so we can show it
e.g. in the Manager interface. This information is available as
a callerid (or something like that) during a call, but not when a
device is registered but silent.
It may be useful to have it available e.g. when developing a user
interface/operator panel, to map numbers to names.

experimental, so not committed to 1.4

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@47093 65c4cc65-6c06-0410-ace0-fbb531ad65f3