Base idle timeout on player->curup again, not current time

Base idle timeout on player->curup again, not current time

Idle timeout used to expire max_idle minutes after the last
player->curup update.  When we got rid of the idle thread in commit
08b94556 (v4.3.20), this got changed to "wait no more than max_idle
minutes for input".  Time spent computing and time spent blocked on
output no longer counts.  In particular, a connection can block
indefinitely on output since then.  Let's fix that.

Start with basing the input timeout on player->curup again.  The
missing output timeout will be added shortly.

Aside: since status() updates player->curup, the idle timer gets reset
when the update aborts a command.  Left for another day.