Fix random segfault when logging.

This only happened in the background, because the error
was in the calling of (v)syslog, not in (v)fprintf.
Program options: change usage message to properly reflect
default bit rate of 500 kbps (not 1000). Also make the listen
option not composite; if a user wants to override the ports
to listen to, he doesn't want to add ports but replace them
from the config.
Changes to encoder control.

- Allow -1 to be used as bitrate (which VU+ advertises to be
"best" (might mean "unconstrained"?).

- Check what encoder to use before opening it.

This allows encoder control options to be set before the
transcoding device is open. Changing options when the device
is already open which might easily confuse it (i.e. crash the stb).

This also introduces a race condition, unfortunately. Just
don't start two transcoding sessions at the same time, it will
crash the driver anyway.