ports-mgmt/portmaster: DEPRECATE without expiration date

The portmaster script hasn't had an official maintainer in 9 months and 2
years before that it was only patched in reaction to changes in the ports
framework. There are many unclaimed PRs in the bugzilla database, many
known bugs, and several areas where portmaster no longer aligns with how
ports work today. The problem isn't simply getting a maintainer; that
person has to be a ports framework expert and it appears that the people
with these qualifications don't want anything to do with this port.

Moreover, there are better options available. All FreeBSD platforms
support ports-mgmt/poudriere (although some many struggle under the load)
and the most common amd64 and i386 platform users have the additional
option of ports-mgmt/synth which is user-friendly, lightweight, and aimed
at users of portmaster, portupgrade, and even poudriere.

Unless something drastic regarding portmaster occurs, it's nearing its
natural EOL, so it's users should evaluate alternatives and try to
migrate off of it.