ssh: Apply CVE-2023-38408 fix from OpenSSH 9.3p2

OpenSSH 9.3p2 provides a fix CVE-2023-38408 - a condition where specific
libaries loaded via ssh-agent(1)'s PKCS#11 support could be abused to
achieve remote code execution via a forwarded agent socket if the
following conditions are met:

• Exploitation requires the presence of specific libraries on the victim system.
• Remote exploitation requires that the agent was forwarded to an attacker-controlled system.

This commit is the primary part of the fix in 9.3p2, applied to the
OpenSSH version in FreeBSD 12.x.

Security: CVE-2023-38408
Sponsored by: The FreeBSD Foundation