Stop sending IPv6 link-local scope-ids in SIP messages

Stop sending IPv6 link-local scope-ids in SIP messages

The idea behind the patch listed below was used, but in a more targeted manner.
There are now address stringification functions for addresses that are meant to
be sent to a remote party. Link-local scope-ids only make sense on the machine
from which they originate and so are stripped in the new functions.

There is also a host sanitization function added to chan_sip which is used
for when peer and dialog tohost fields or sip_registry hostnames are used to
craft a SIP message.

Also added are some basic unit tests for netsock2 address parsing.

(closes issue ASTERISK-17711)
Reported by: ch_djalel
Patches:
      asterisk-1.8.3.2-ipv6_ll_scope.patch uploaded by ch_djalel (license 1251)

Review: https://reviewboard.asterisk.org/r/1278/

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@324484 65c4cc65-6c06-0410-ace0-fbb531ad65f3