Fix daemon.py tests' dependence on netcat

This commit fixes two intertwined problems:

1. with traditional netcat, netcat -l 8888 treats "8888" as a host, and never starts listening on port 8888.

   This is fixed by replacing netcat with a Python script (as suggested by Lars Wirzenius);

2. Subplot's test suite never notices the above problem because, even though it fails to connect to the port, this error is intentionally swallowed by _daemon_start_soonish().

   This is fixed by adding an explicit check to ensure that the daemon started after all. Furthermore, the scenario description is reworded to make it clear that we do expect the daemon to start, it'll just take a while.