ssh: drop $FreeBSD$ from crypto/openssh

After we moved to git $FreeBSD$ is no longer expanded and serves no
purpose. Remove them from OpenSSH to reduce diffs against upstream.

Sponsored by: The FreeBSD Foundation

Unbreak the WITHOUT_KERBEROS build and try to reduce the odds of a
repeat performance by introducing a script that runs configure with and
without Kerberos, diffs the result and generates krb5_config.h, which
contains the preprocessor macros that need to be defined in the Kerberos
case and undefined otherwise.

Approved by: re (marius)