Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Add a script to create the /var/db/mergemaster.mtree file for new
releases so that when users subsequently update their source trees
they can make use of mergemaster's -U option.