Use fd signal trick to break us out of select; do not sigwait

Use fd signal trick to break us out of select; do not sigwait

* The sigwait approach to detecting aio does not work because some
  versions of glibc forget to block signals on the private aio thread.
  This means that blocking SIGUSR2 is ineffective; the signals can be
  lost and the program can block in sigwait (!)

* Use of SIGUSR2 to interrupt select() does not work because signals
  which arrive just before entry to select() do not interrupt it.

So instead we use the time-honoured self-pipe trick: in the signal
handler we write to a pipe, which we select on when we want to wait
for the signal, and which we read from (to empty out) just before
actually doing the `top half' processing which deals with the condition
to which the signal relates.