Ability for maintainers to update own ports

[Mark Linimon]

It's always a little dicey to follow up to oneself, but here goes:

> There is by no means a shortage of orphaned ports.  Everyone
> should adopt one today! :-)

I tweaked my ports monitoring code, and it now generates the following
statistics:

Total number of ports: 9629
Number of ports with no maintainer: 2519 (26.2%)
Ports with no maintainer and build errors in at least one build environment: 
423 (4.39%)
Ports with no maintainer and outstanding PRs: 135 (1.40%)
Union of the above two ("ports in need of attention" in my shorthand): 520
(5.40%)

So there's some statistics for folks to ponder about.  If you want to help
FreeBSD's ports collection, that "423" number surely looks like a good
place to start.

mcl