Move the signal information out of the signal pipe.

Move the signal information out of the signal pipe.

The signal pipe included all signal information.  This reduces the
number of signals that can be dispatched in a single event loop
iteration.  It also limits the amount and/or type of information we
can include in the signal.

This change puts the signal information in a plain Perl list and only
uses the signal pipe to wake up the event loop to dispatch the signals
in a safer place.