Create systemd-{network,resolve} user/group unconditionally

This shuts up this error from dbus:

May 11 13:52:16 machine dbus-daemon[259]: Unknown username "systemd-network" in message bus configuration file
May 11 13:52:16 machine dbus-daemon[259]: Unknown username "systemd-resolve" in message bus configuration file

which happens because the D-Bus config for networkd/resolved is
enabled unconditionally, and we don't have an easy way to turn it off.

(cherry picked from commit f19b58fb)