systemd: add --version option

systemd --version mirrors systemctl --version:

$ ./systemd --version
systemd 186
other
+PAM +LIBWRAP +AUDIT +SELINUX +IMA +SYSVINIT -LIBCRYPTSETUP

This information can be retrieved by other means (systemctl, etc.),
but it's easier for a newbie if 'systemd --version' says something
useful. And 'systemd --help' is already there, so let's complement
that with '--version'.