Using an empty listening address will result in chasquid listening on a
random port, which is a dangerous misconfiguration.
That is most likely done to prevent it from listening at all.
To prevent this misconfiguration, explicitly reject empty listening
addresses early and with a warning, so there is no ambiguity.
Users can still prevent chasquid from listening by just commenting out
the entry in the config (and not passing any systemd file descriptors).
See https://github.com/albertito/chasquid/issues/45 for more details and
discussion, including alternatives considered.
Thanks to xavierg who reported this via IRC.
The t-20-bad_configs test can sometimes have false positives because
the last line recorded in the log isn't always the one causing the
failure, due to asynchronous logging and in particular in combination
with coverage tests (which alter the os.Exit behaviour subtly).
This patch fixes that by having the tests look at the last 4 lines of
output instead. This is not super pretty, but it should be good enough
to cover for any timing issues that arise in these particular tests.
