Make portsnap off by default

[swills]

Probably incomplete, please let me know what I'm missing.

[cperciva]

I'm ok with this going into 14.0 as long as there's no intention to MFC; however, I think as a "deprecation path" we should probably consider putting portsnap back into the ports tree (as we've done many times in the past when code is removed from base), at least temporarily.

[swills]

In D31437#710938, @cperciva wrote:

I'm ok with this going into 14.0 as long as there's no intention to MFC; however, I think as a "deprecation path" we should probably consider putting portsnap back into the ports tree (as we've done many times in the past when code is removed from base), at least temporarily.

Thanks. Adding portsnap to ports sounds reasonable. To that end, I've created D31543, but it will probably need some work before it can go in.

Also, I'm not a src committer, so I'll need someone to commit this diff.