The orphan responses matching with the dialog can now be processed by the
response callback.The dialog leg can be created with
NTATAG_RESPONSE_CALLBACK() or a response callback can be later bound to the
leg with nta_leg_bind_response().

This is practically useful only with 200 OK responses to the INVITE that are
retransmitted by the UAS. By default, the retransmission are catched by the
ACK transaction (which then retransmits the ACK request message). However,
after ACK transaction times out, the retransmitted 200 OK indicates most
probably that the ACK request messages do not reach UAS.

Partially fixes the sf.net bug #1750691 reported by Mikhail Zabaluev.

darcs-hash:20070824134120-65a35-b75a200339bb5dfd3057b4528334098e69935e5b.gz