sipreg: add Contact rewrite from Via received/rport

[qfiber]

Behind a NAT the local address in the REGISTER Contact is not reachable from the registrar. Registrars that rewrite the Contact from the request's source hide this; with the rest, requests to the registered contact never arrive.

This adds an opt-in Contact rewrite to sipreg, based on the address the registrar reports in the received and rport parameters of the 2xx's top Via (RFC 3261 18.2.1, RFC 3581):

• sipreg_set_contact_rewrite(reg, true) enables it (off by default, so nothing changes for existing users).
• When a 2xx shows an address that differs from the one in our Contact, the client sends a new REGISTER with that address, and removes the stale binding with a second Contact: ...;expires=0 in the same request.
• The response handler is only called for the final response, so the application sees one successful registration.
• Two addresses taking turns (multiple NAT mappings) are capped at two consecutive rewrites.
• sipreg_contact_addr() returns the address currently in the Contact (the learned one when rewritten, otherwise laddr), so an application can use it in the Contact of its dialogs as well. A baresip PR using this follows.

The test puts the mock NAT (NAT_INBOUND_SNAT) on the mock registrar's UDP socket (via the new sip_transp_udp_sock()), so the registrar sees the client at 192.0.2.10, and checks the second REGISTER: the stale local binding with expires=0, then the public one. It fails without the rewrite enabled. retest -r passes, also under ASan, and ccheck is clean.

Tested against Asterisk 22 behind a home NAT: the client registers its private address, re-registers with the public one and removes the private binding.

[qfiber]

Thanks for the review, and sorry for the extra work. I'm new to contributing here, so apologies for the mistakes.

All done in 7e4fd4c: renamed to rewrite_contact(), removed the comments on the static helpers, added the RFC sections to the sipreg_set_contact_rewrite() doxygen, and switched the test comment to doxygen style.

[cspiel1]

Looks good now.