This makes it easier to actually test what happens when channel numbers wrap
around.  The good news: it works.

However, I did find a bug where sshuttle would die if we completely ran out
of available channel numbers because so many of them were open.  This would
never realistically happen at the default of 65535 channels (we'd run out of
file descriptors first), but it's still a bug, so let's handle it by just
dropping the connection when it happens.