systemd/pmos-base-systemd: disable multipathd by default

This causes systemd to get stuck restarting multipathd forever on boot if it runs on systems with no multipath hardware. When this happens, the system never finishes booting.

It doesn't seem like any UIs/devices enable the multipathd service by default in openrc, and that we just depend on multipath-tools for kpartx and stuff (and not the daemon that comes with it)