Implement "strict key exchange" in ssh and sshd.

This adds a protocol extension to improve the integrity of the SSH
transport protocol, particular in and around the initial key exchange
(KEX) phase.

Full details of the extension are in the PROTOCOL file.

OpenBSD-Commit-ID: 2a66ac962f0a630d7945fee54004ed9e9c439f14

Approved by: so (implicit)
Obtained from: https://anongit.mindrot.org/openssh.git/patch/?id=1edb00c58f8a6875fad6a497aa2bacf37f9e6cd5
Security: CVE-2023-48795