Connection utilities

Arm's mostly-cross-platform connection utilities are one of its most used features. We should migrate those utilities (with tests!) to stem to make them available to other applications...

https://gitweb.torproject.org/arm.git/blob/HEAD:/src/util/connections.py

Bonus points if we add OpenBSD support when we do this (mischief had some suggestions for this on #7813 (closed)).