Swift 3.1

[lichtblau]

Port to Swift 3.1 (tested with the swift-3.1-DEVELOPMENT-SNAPSHOT-2017-03-19-a-ubuntu16.10 tarball).

In this branch:

• Fixing the build error is a whitespace change... This is the one and only commit that actually matters.
• Various super important warnings being addressed. Hope this doesn't break anything. Tests pass.
• Unrelated semicolon change after touching that file...

Not in this branch:

• Something about pointer arithmetic remains. Fixable, but I've got more questions than answers about that code.
• readdir_r is apparently pointless and should be replaced with readdir
• Warnings in tests could also be addressed.
• Need to update the dgram copy&paste files similarly.

There are several ways to quiet down swift on the string interpolation of optionals. I've opted for the "as Optional" instead of ".debugDescription" for no big reason. Both seem roughly equally ugly.

[helje5]

OK, I'm going to check it out tomorrow. I assume this should not affect Swift 3.0.2 builds, right? (I see Travis is passing, but still).

[helje5]

readdir_r is apparently pointless

The web says it is 'harmful' ;->

The macOS man page of readdir doesn't say whether it is thread safe or not (the Linux one is for our purposes).
While we are only on one thread right now, I think we should keep the doors open to run on multiple accept queues in one process.