Separate login_grace_time from max_idle

Separate login_grace_time from max_idle

max_idle applies in state PS_PLAYING, login_grace_time before (login,
state PS_INIT) and after (logout, state PS_SHUTDOWN).

Cut login_grace_time to two minutes, from max_idle's 15.  Two minutes
is plenty to complete login and logout.  Makes swamping the server
with connections slightly harder, as they get dropped faster.  While
that makes sense all by itself, the real aim is making increasing
max_idle safe.  The next commit will complete that job.