core/swap: follow the configured unit by default

core/swap: follow the configured unit by default

Phenomenon: parameters configured in /etc/fstab for swap units are
ignored. E.g. pri= settings have no effect when systemd starts swap
units. What is even more confusing, .swap units for the name used in
/etc/fstab initially show proper values for Priority=, but after
starting them, they are re-initalized from /proc/swaps and show the -1
value from /proc/swaps.

Change swap units to follow the original configured unit. This way
proper settings are used when starting the swap.