fuse: Set mtime on /connection to logged_in_time

Also, we don't need the boolean logged_in if we can treat the
logged_in_at variable to mean a real value or just be 'false'
(time-wise).

Anton: feel free to wreak havoc, globals are bad in c as
well, right?

Signed-off-by: Carl Helmertz <helmertz@gmail.com>