homed: keep "pinning" fd open while home dir active

homed: keep "pinning" fd open while home dir active

The pin fd keeps the mount busy, ensuring that unmount requests need to
go through us.

Note that this doesn't change too much IRL, since a logged in user
generally has processes keeping the home dir busy anyway. However, in
some corner cases it is safer to protect from accidental unmounts this
way. (e.g. if user manually called "homectl activate" first).