Add attended transfer support for chan_sip.c

This now uses the core API for performing attended transfers.

Review https://reviewboard.asterisk.org/r/2513

(Closes issue ASTERISK-21520)
reported by Matt Jordan



git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@389869 f38db490-d61c-443f-a65b-d21fe96a405b
Add missing NULL check to acquire_bridge() function.



git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@389870 f38db490-d61c-443f-a65b-d21fe96a405b