homed: add bus call that allows referencing a home without activating it

homed: add bus call that allows referencing a home without activating it

This is useful for allowing users to login without the ability to unlock
their home dir. Usecase is ssh: ssh might grant access without giving
us the chance to unlock the home dir for the user (because it doesn't
allow us asking questions during authentication), hence with this call
we can pin the home dir, but not activate it and then allow the
activation to be delayed until later.