input_pipe: do not loop in ->event_step for fairness

input_pipe: do not loop in ->event_step for fairness

Sigfd->event_step needs priority over InputPipe (and everything
else).  We keep Edge Triggering here but use ->requeue instead
of looping inside event_step.  This was necessary because
InputPipe can be used with regular files which can't be
monitored with epoll.

We'll also rid of the vestigial lei-oneshot support while we're
at it.