PJSIP: Allow use of 'inactive' streams for hold

This allows use of the 'inactive' stream direction identifier to be
used for hold where 'sendonly' is normally used. Some Seimens phones
use 'inactive' and this change allows music on hold to operate
properly.

Review: https://reviewboard.asterisk.org/r/4252/
Reported by: Steve Pitts


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@429432 f38db490-d61c-443f-a65b-d21fe96a405b