Add a customizable log handler

Add a customizable log handler

The previous log handler wasn't actually hooked up to anything. Add a public
API for the log handler with priority filtering, defaulting to priority
'error' and stderr as output stream.

And to keep the diff down and convenience up, provide a few simple wrappers
for logging. The generic is log_msg(), but let's use log_info, log_error, etc.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>