ck-log-system-start: always create log directory

Some embedded setups (like OLPC) don't disk-back /var/log (e.g. enabled
by Fedora's readonly-root functionality), so this will be empty on
every boot.

In this setup, ck-log-system-start fails because /var/log/ConsoleKit
doesn't exist.

Fix this by copying in mkdir code from ck-event-logger.c:open_log_file
which already correctly handles this situation. Reported in:
http://lists.freedesktop.org/archives/consolekit/2011-September/000148.html