homed: add a ActivateHomeIfReferenced() bus call

homed: add a ActivateHomeIfReferenced() bus call

This is very similar to ActivateHome() but will fail if the home
directory is not referenced yet. Or in other words, this doesn't add any
new reference, but simply is the other side of RefUnrestricted(): if we
allowed a home dir to be referenced without it actually being active,
then this can catch up with things and activated what was previously
referenced already.

This also relaxes access rights to that users can always activate their
own home dirs. This is useful once we allow user code to run without the
home dir being activated.