sip: UDP for the protected leg, TCP first with a fallback

imsd's protected leg (the second REGISTER and everything after it) was
TCP only. A P-CSCF that never answers the TCP connect on its protected
server port (O2 UK: the SYNs leave ESP-protected, nothing comes back)
left the unit looping with no way forward, although the SAs, the
listener sockets and the firewall rule already covered UDP.

The client flow now carries a transport. UDP binds the same protected
client port and connect()s the datagram socket to the P-CSCF's protected
server port, so the kernel delivers that peer's datagrams to it ahead of
the unconnected listener on the same port. One datagram is one message
(RFC 3261 18.3: a Content-Length that fits truncates, one that does not
fit discards the datagram, none means the rest of the datagram); a
receive error is logged and marks the flow dead; a message over the
single-packet ESP budget at the ims PDN's MTU is logged once per flow,
since the kernel fragments it and a P-CSCF may drop the fragments.
Every protected request's Via follows the transport; the challenge
stays UDP.

SIP_TRANSPORT selects the policy: auto (default) registers over TCP and,
after two unanswered connects, registers again over UDP (new challenge,
new SA pair) - but only on a phone whose state file does not record a
successful TCP registration, so an outage on a TCP carrier never becomes
a second initial REGISTER; a phone whose last registration was UDP goes
straight to UDP. tcp and udp force one. A connect refused locally
(EADDRNOTAVAIL: the 4-tuple still in TIME_WAIT from the previous flow)
keeps retrying and is not counted as silence; SO_ERROR results are
logged by name. The registration's transport is persisted so a warm
resume reconnects the same way, and the resume gives up after two
silences.

Two things the change exposed and fixes: an engine-fatal event exited 0
("exiting for systemd restart" with Restart=on-failure never firing),
and a fresh registration refused with the P-CSCF's fresh-SA throttle
(Security-Server spi-s=0) must not be retried by a 120-s restart loop,
since every attempt re-arms the ~20-min window - it is now waited out
in-process, growing on repeats, with D-Bus commands still served. A
resume the network refuses falls through to a fresh registration. A
retransmitted 200 OK to our INVITE is ACKed again and no longer starts
a second media leg.

Verified on KPN: resume over TCP after a binary swap, MT and MO calls
with media both ways, the throttle deferral and its self-recovery, the
UDP client path up to KPN dropping the datagram. The UDP success path
is a field test; SIP retransmission timers over UDP are not in this
change.

tests/Sip/main.cpp

@@ -174,6 +174,28 @@ int main() {
         Check(!ViaResponsePort("UPDATE sip:me SIP/2.0\r\n\r\n").has_value(), "no Via -> nullopt");
     }
 
+    // ---- DatagramMessage (one UDP datagram = one message, RFC 3261 18.3)
+    {
+        std::string why;
+        Check(!DatagramMessage("\r\n", &why).has_value() && why.empty(), "CRLF-only datagram is a pong, not a message, no problem");
+        Check(!DatagramMessage("\r\n\r\n\r\n").has_value(), "CRLFCRLF-only datagram is a pong");
+        std::string hdr = "SIP/2.0 200 OK\r\nCall-ID: a\r\n";
+        auto m0 = DatagramMessage(hdr + "Content-Length: 0\r\n\r\n");
+        Check(m0 && *m0 == hdr + "Content-Length: 0\r\n\r\n", "Content-Length 0, no body: whole datagram");
+        auto m1 = DatagramMessage(hdr + "Content-Length: 4\r\n\r\nv=0\nJUNK");
+        Check(m1 && *m1 == hdr + "Content-Length: 4\r\n\r\nv=0\n", "Content-Length that fits truncates the body");
+        why.clear();
+        Check(!DatagramMessage(hdr + "Content-Length: 40\r\n\r\nv=0\n", &why).has_value() && why.contains("truncated"), "Content-Length larger than the datagram: discarded, problem set");
+        auto m2 = DatagramMessage("\r\n" + hdr + "\r\nv=0\n");
+        Check(m2 && *m2 == hdr + "\r\nv=0\n", "no Content-Length: body is the rest of the datagram, leading CRLF stripped");
+        why.clear();
+        Check(!DatagramMessage("SIP/2.0 200 OK\r\nCall-ID: a\r\n", &why).has_value() && why.contains("terminator"), "no header terminator: discarded, problem set");
+        auto m3 = DatagramMessage(hdr + "X-Note: Content-Length: 99\r\n\r\nv=0\n");
+        Check(m3 && *m3 == hdr + "X-Note: Content-Length: 99\r\n\r\nv=0\n", "a header name inside a value does not match");
+        auto m4 = DatagramMessage(hdr + "Content-Length: 99999999999999999999\r\n\r\nv=0\n", &why);
+        Check(!m4.has_value(), "absurd Content-Length: discarded, no overflow");
+    }
+
     if (Failures == 0) std::println("Sip: all tests passed");
     return Failures;
 }