(pr_player, upr_player): Blocking I/O on another player's struct iop *

(pr_player, upr_player): Blocking I/O on another player's struct iop *
is unsafe!  By the time the blocked thread wakes up, that player may
be gone, along with its struct iop *, and io_write() follows a
dangling pointer.  Moreover, at most one thread may use empth_select()
on the same file descriptor.  Violations of that restriction cause
threads to hang under Windows since ntthread.c rev. 1.15.  Make all
output to another player non-blocking for now.  Historically, player
threads sent output only to their socket, though their own iop.  This
was broken by flash and asynchronous telegram notification a long time
ago.