Don't log out player when update aborts a command with pthreads

Don't log out player when update aborts a command with pthreads

pthread.c's empth_select() returned -1 when empth_wakeup() interrupted
select().  The failure then got propagated all the way up, and the
player got logged out.  Fix by returning 0 in that case.  While there,
retry on EINTR, to match LWP.  Also clarify comments.