daemon: improve session closure readability

daemon: improve session closure readability

Until now, sessions were closed by explicitly sending `_CLOSE` events
via the `session2_event()` function to them, which I think is not
signalling the intent very well. It might look as though the session
has been/is being closed by some part of the code that contains the
`session2_event()` call and a relevant event is being fired now. This
commit introduces `session2_close()` and `session2_force_close()` inline
functions, which do the same thing, but I think the intent behind
calling them is slightly clearer.