Factor out startup logic from init script

Separate init script startup logic into the following parts:

 - modules are now loaded via /lib/modules-load.d
 - sanity checks are now done in /lib/open-iscsi/startup-checks.sh
 - activating the storage after login to the iSCSI targets is now done
   in /lib/open-iscsi/activate-storage.sh

These scripts may now be called from native systemd services as well as
the init script.

Network filesystems are now also only mounted on non-systemd systems,
since systemd activates mounts when devices newly appear by itself, so
it's not needed there.

Multipath is now explicitly supported to anticipate the potential
removal of the udev rule (which is the only thing making the current
setup work).

HANDLE_NETDEV in /etc/default is now actually respected, previously
that was only the case at shutdown.