res_pjsip_multihomed: Make address replacement less aggressive.

This change makes the res_pjsip_multihomed module less aggressive when
changing the address in messages. It will now only occur if the transport
in use is bound to the any address OR if the system determined source
address matches the bound address of the transport in use.

Review: https://reviewboard.asterisk.org/r/3369/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@410793 f38db490-d61c-443f-a65b-d21fe96a405b
res_pjsip: Enable PJSIP DNS client support.

This change enables DNS client support within PJSIP. System
nameservers are automatically discovered using res_init or
res_ninit. If this fails then PJSIP will resort to using
gethostbyname for resolution.

By enabling this support we gain SRV support, failover, and
weight support.

(closes issue ASTERISK-23435)
Reported by: Matt Jordan

Review: https://reviewboard.asterisk.org/r/3343/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@410795 f38db490-d61c-443f-a65b-d21fe96a405b