Debugger[New CDB]: Fix stepping issues, forced shutdown, values.

- Do not report start-up trap as stop reason should one trigger
  while stepping.
- Set current frame to first usable frame, avoiding disassembly
  while stepping
- Kill process in case of forced shutdown with pending (long)
  commands.
- SymbolGroupValue: Do a quick check for accessible values
  and mark failure as 'invalid'.
- Introduce flag indicating existence of simple dumpers to types.