This adds support for reading log entries from the Systemd journal. A
new command, called simply 'journal', is added which takes a number of
lines as a parameter and optionally which journal (user or system) to
read from.
Obviously, libsystemd is required to build journal support. Tested on
Arch Linux (Systemd v229).
Signed-off-by: Toke Høiland-Jørgensen <toke@toke.dk>
