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
$FreeBSD$ tags and man page dates.
Add a post-merge script which reapplies these changes.
Run both scripts to normalize the existing code base. As a result, many
files which should have had $FreeBSD$ tags but didn't now have them.
Partly rewrite the upgrade instructions and remove the now outdated
list of tricks.