res_pjsip: Support setting a default accountcode on endpoints

Most channel drivers let you specify a default accountcode to be set on
channels associated with a particular peer/endpoint/object. Prior to this
patch, chan_pjsip/res_pjsip did not support such a setting.

This patch adds a new setting to the res_pjsip endpoint object, 'accountcode'.
When a channel is created that is associated with an endpoint with this value
set, the channel will automatically have its accountcode property set to the
value configured for the endpoint.

Review: https://reviewboard.asterisk.org/r/3724/

ASTERISK-24000 #close
Reported by: Matt Jordan


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@418756 f38db490-d61c-443f-a65b-d21fe96a405b