core: add LUOSession= unit setting (#42530)

Acquiring a LUO session from /dev/liveupdate requires privileges, and
also the device is a single-owner driver so only a single process can
open it at any given time.

Add a LUOSession= service settings that allows units running without
privileges to get a session assigned to them.

The kernel imposes a 64 chars limit on session names, which is too short
to avoid clashes, so derive a hash from joining the unit name with the
parameter name, that way two units using the same setting don't clash.