[Asterisk-Dev] SIPGetHeader now deprecated

[Kevin P. Fleming]

To start the process of what was discussed during the dev call last 
week, I've added a SIP_HEADER function and marked SIPGetHeader as 
deprecated. I've scheduled it to be removed at the end of this month.

Potentially the SIP_HEADER function could be writable as well, but the 
semantics would not quite be the same as SIPAddHeader, so I've not done 
that yet.