Disconnect the other call leg when multiple 2xx/OK responses to INVITE are received due to forking

[bennylp]

Currently when more than one 200/OK are received by UAC due to forking, the second one will be ignored since pjsua-lib doesn't support this.

A cleaner solution is to send BYE to the second call leg if the application is not able to handle multiple call leg.

[bennylp]

Done in r2315:

• Implement on_dlg_forked() of the UA layer module to create the forked dialog for the forked 2xx response. This forked dialog is only used to send BYE request for the call leg.
• Added hangup_forked_call setting in pjsua_config to specify whether forked calls should be hung up. Default is yes. Only disable this feature for testing, since pjsua-lib doesn't support forked media.
• Some fixes in the sip_dialog.c:
  • fixed bugs in pjsip_dlg_fork(): target and route-set are not initialized correctly from the forked response.
  • handle authentication of the request (e.g. outgoing BYE) since when no invite session is created for the dialog.
• Some fixes in the sip_ua_layer.c:
  • ignore the response when on_dlg_forked() callback returns NULL. This could happen when forked dialog initialization fails.

Note that this changeset depends on r2314 which fixed a bug in pjsip_auth_clt_clone()