journalctl: add "-t --identifier=STRING" option

This turns journalctl to the counterpart of systemd-cat.
Messages sent with

systemd-cat --identifier foo --prioritiy debug

can now be shown with

journalctl --identifier foo --prioritiy debug

"--identifier" is not merged with "--unit" to make a clear
distinction between syslog and systemd units.
syslog identifiers can be chosen freely by anyone.