peerflix: Use XDG cache dir by default

Instead of peerflix' default /tmp.
peerflix: Use a random unused port by default

Instead of hard-coding 10000 as the port, which would break
concurrent uses.
peerflix: Remove race condition for port selection on non-Linux

The Internets suggest that Node.js does use SO_REUSEADDR. Can be
revisited if this turns out not to be the case (since I don't have a
machine to test this on).