core: add LUOSession= unit setting

core: add LUOSession= unit setting

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.