Use ast_sockaddr_stringify_fmt wrappers for various functions in chan_sip

Use ast_sockaddr_stringify_fmt wrappers for various functions in chan_sip

There are a number of cleaner looking wrappers for ast_sockaddr_stringify_fmt
available which are slightly more readable than using a direct call to
ast_sockaddr_stringify_fmt. This patch switches a number of those calls in
chan_sip to use those wrappers and is generally harmless.

(Closes issue ASTERISK-16930)
Reported by: Michael L. Young
Patches:
	chan_sip-broken-registration-1.8.diff uploaded by Michael L. Young (license 5026)

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@353720 65c4cc65-6c06-0410-ace0-fbb531ad65f3