When checking of a 'foot --server' instance is already running, we try
to connect to the UNIX socket we're planning on listening on.
In most cases, this will fail hard and fast. But under certain
circumstances, we can get stuck in connect() waiting for a connection
timeout.
Since it should be possible to establish a connection immediately *if*
there's someone actually listening on the socket, rely on the fact
that connect() will fail with a EINPROGRESS if a connection can *not*
be established immediately (and thus no one is listening on it).
In this mode, foot listens on a UNIX socket and creates terminal
windows when clients connect.
A connecting client sends argc/argv to the server, and the server
instantiates a new terminal window.
When the terminal window is closed, the exit code is sent back to the
client.
