MEDIUM: log: support a new "short" format

MEDIUM: log: support a new "short" format

This format is meant to be used with local file descriptors. It emits
messages only prefixed with a level, removing all the process name,
system name, date and so on. It is similar to the printk() format used
on Linux. It's suitable to be sent to a local logger compatible with
systemd's output format.

Note that the facility is still required but not used, hence it is
suggested to use "daemon" to remind that it's a local logger.
Example :

    log stdout format short daemon          # send everything to stdout
    log stderr format short daemon notice   # send important events to stderr