add portsnap to the ports tree

[swills]

This is just a first pass.

This is just a direct copy of the files from src. Maybe there's a better way. One thing I considered was a repo under https://github.com/freebsd/ similar to poudriere and drm-kmod.

I wasn't sure about who should maintain this, perhaps @cperciva should, but put myself for now.

I also wasn't sure about the version number, so just picked today's date.

There are a few places where there are trailing spaces at end of line that could be cleaned up.

Perhaps the code should be changed to read ${PREFIX}/etc/portsnap.conf in addition to /etc/portsnap.conf.