This logging is available if the software was build with the configure
option --with-systemd-journal.
It will be enabled by default if libsystemd is found.
The runtime parameter --log-journal enables to output of log messages
to the systemd journal.
Please note that the journal priority field has the same value as the
syslog level so no conversion is required here.
Group together all options that allow configuring the logging output,
and make them have the same --log-[XXX] prefix.
Also rework the --help output so that all option groups are printed by
default (i.e. there is no longer a --help-all option).
Standard GPS setup (raw/nmea) will both enable the GPS module and take full
control of the GPS port. This prevents other processes from reading the NMEA
traces from e.g. a tty. In order to handle this, a new 'unmanaged' GPS location
source is introduced, which will just enable/disable the GPS module, without
reading anything from the GPS port. Of course, both raw/nmea and unmanaged
setups cannot be enabled at the same time.
