nta.c: added timer N3.
nta.c: added timer N3.

Timer N3 is used to guard against the excessive long delays when
establishing a TCP connection. If a SIP request message is over 1300 bytes
the stack tries to use TCP even if UDP is specified. Without N3 the
transaction times out if the stack receives no response to TCP SYN segment
(connection establishment request) because, e.g., a "stealth-mode" firewall
drops the SYN segment. The "stealth-mode" firewall does not respond with a
ICMP unreachable packet or RESET segment.

Thanks for Michael Jerris, Johan Paul, Sami Sirkia and Jarmo Ruusiala for
repeatedly reporting the problem.

darcs-hash:20061205163928-65a35-d8fdf51506c365f5f43987e882571a5895aeb29a.gz

unispeech/sofia-sip,xhook/sofia-sip,erdincay/sofia-sip,BelledonneCommunications/sofia-sip,xhook/sofia-sip,xhook/sofia-sip,unispeech/sofia-sip,unispeech/sofia-sip,BelledonneCommunications/sofia-sip,unispeech/sofia-sip,BelledonneCommunications/sofia-sip,xhook/sofia-sip,BelledonneCommunications/sofia-sip,jart/sofia-sip,jart/sofia-sip,erdincay/sofia-sip,erdincay/sofia-sip,jart/sofia-sip,xhook/sofia-sip,erdincay/sofia-sip