pam_krb5: Fix spoofing vulnerability

An adversary on the network can log in via ssh as any user by spoofing
the KDC. When the machine has a keytab installed the keytab is used to
verify the service ticket. However, without a keytab there is no way
for pam_krb5 to verify the KDC's response and get a TGT with the
password.

If both the password _and_ the KDC are controlled by an adversary, the
adversary can provide a password that the adversary's spoofed KDC will
return a valid tgt for. Currently, without a keytab, pam_krb5 is
vulnerable to this attack.

Reported by: Taylor R Campbell <riastradh@netbsd.org> via emaste@
Reviewed by: so
Approved by: so
Security: FreeBSD-SA-23:04.pam_krb5
Security: CVE-2023-3326