journalctl,loginctl: drop systemd- prefix in binary names

Let's make things a bit easier to type, drop the systemd- prefix for
journalctl and loginctl, but provide the old names for compat.

All systemd binaries are hence now prefixed with "systemd-" with the
exception of the three primary user interface binaries:

systemctl
loginctl
journalctl

For those three we do provide systemd-xyz names as well, via symlinks:

systemd-systemctl → systemctl
systemd-loginctl → loginctl
systemd-journalctl → journalctl

We do this only for the *primary* user tools, in order to avoid
unnecessary namespace problems. That means tools like systemd-notify
stay the way they are.