Add a pre-merge script which reverts mechanical changes such as added
$FreeBSD$ 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Partly rewrite the upgrade instructions and remove the now outdated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.
Add a note to UPDATING about customized kernel configuration files now
requiring -gdwarf-2 for debug info, when using clang 3.4.

Suggested by:	adrian
MFC after:	3 days