nta.c creates a ACK transaction in order to restransmit ACK requests when
ever a retransmitted 2XX response to INVITE is received. The UAS retransmits
the 2XX responses for 64 x T1 (32 second by default).

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

darcs-hash:20070824133535-65a35-d7f6828721fdbce4e45569a9c4a05cc0ded741c0.gz