res_pjsip: Enable PJSIP DNS client support.

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: https://origsvn.digium.com/svn/asterisk/branches/12@410795 65c4cc65-6c06-0410-ace0-fbb531ad65f3