Fix player thread race with update_init()

Fix player thread race with update_init()

start_server() creates the thread running player_accept() before it
calls update_init().  However, update_init() initializes stuff used to
player threads: update_time[] and play_lock.  In theory, a player
thread could start before that, and crash when taking the
uninitialized play_lock.

Delay starting that tread until after update_init().