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.

README.md

@@ -161,6 +161,7 @@ PCSCF=2001:db8::105
 | `RESUME` | `auto` | `0` forces a fresh registration (ignores a warm SA) |
 | `REFRESH_INTERVAL` | *(auto)* | keepalive re-REGISTER period in s; default = half the granted expiry, clamped to [120, 1800] |
 | `EALG` | `aes-cbc` | offered ESP cipher (`aes-cbc`, `des-ede3-cbc`, `null`) |
+| `SIP_TRANSPORT` | `auto` | transport of the protected leg (the second REGISTER and everything after it): `tcp`, `udp`, or `auto` = TCP, falling back to a fresh registration over UDP when the P-CSCF leaves two TCP connects unanswered — but only on a phone that has never registered over TCP: once the state file (`/var/lib/imsd/imsreg.state`) records a TCP registration, a silent connect is treated as an outage, not a policy (delete the file or set `udp` to override, e.g. after a SIM change). The challenge (first REGISTER) is always UDP. Over UDP a REGISTER or INVITE larger than ~1.2 KB leaves as IPv6 fragments at the ims PDN's 1280-byte MTU (the journal says so once per flow); there are no SIP retransmission timers over UDP yet, so a lost datagram costs a timeout. The SAs, the listener and the firewall rule cover both transports |
 | `EMERGENCY_NUMBERS` | *(empty)* | comma-separated additions to the builtin 112/911 emergency numbers (e.g. a private test core's short code). SIM `EF_ECC` is not read yet |
 | `RTP_PORT` | `50004` | local RTP port advertised in SDP |
 | `PRECOND` | `0` | `1` offers SDP QoS preconditions |