Fix a problem when a Haskell process is suspended/resumed using shell
job control in Unix.  The shell tends to put stdin back into blocking
mode before resuming the process, so we have to catch SIGCONT and put
it back into O_NONBLOCK.

Also:

  - fix a bug in the scheduler: reverse the order of the check
    for pending signals and the call to awaitEvent to block on I/O.

  - do a style sweep in Signals.c