Remove SignalDelivery since it duplicates the enum in Event.h. Make various places use SignalDeterministic instead of bool.
Rename `sigtype` to `deterministic`.
Trigger emulate_signal_delivery from its own TSTEP state.

We want to refactor replay so that every interaction with the debugger
occurs between steps of the replay state machine. This means we need
to remove the synchronous send-debugger-stop-and-continue code currently
in emulate_singal_delivery.
Remove unused 'dbg' and 'req' parameters.
Make sigill a BASIC_TEST.
Add a test to ensure that taking a checkpoint at signal delivery works.