res_pjsip_multihomed: Add module which places the correct address within messages.

Due to how messages are handled within PJSIP it is not until a message is actually
sent that the destination is reliably known. This means that the addresses placed
within the message may not be of the interface the message is being sent out on.

This module determines what interface a message is being sent on and updates the
message to contain the correct address if applicable.

This module was tested by myself in a virtualized environment with multiple interfaces
and also by Kinsey Moore in the following configuration:

Networks:
* 10.24.16.0/21
** hard phone
** default gateway
* 10.24.64.0/21
** softphone with pjsip-based stack

Transport details:
bind address: 0.0.0.0
protocol: UDP

All endpoints were tested with explicitly configured transports and unconfigured transports.

This was tested with inbound and outbound calls, both of which were experiencing detrimental
effects from incorrect IP addresses in SIP messages. These effects were only experienced by the
soft phone on the 10.24.64.0 network since the messages to the hard phone on the 10.24.16.0
network had the correct IP address.

(closes issue ASTERISK-23020)
Reported by: xrobau

Review: https://reviewboard.asterisk.org/r/3102/


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@410451 f38db490-d61c-443f-a65b-d21fe96a405b