src/stackprof.cxx: fixups in prior

src/stackprof.cxx: fixups in prior

- clog/cerr division is ok
- mainfile can also be null
- if we deliberately limit maxframes<=2, the heuristic
  flagging fewer than 2 frames as a 'lost' sample is inapplicable

and

- opt_maxframes relates to the user deliberately choosing to
  override GprofUnwindSampleConsumer preference with --maxframes
  (which I consider useful sometimes for testing):
  if --maxframes is provided, then use the default of 1.