Changes

Summary

1. merged
2. more robust: process stdout on Windows may contain CR;
3. clarified signature; more inlined protocol messages;
4. prefer system option: easier to make it default;
5. support multiple sessions, notably for "isabelle build -P -j2";
6. pretty formatting as in Isabelle/ML;
7. output prover messages;
8. clarified signature;
9. clarified signature: more robust;
10. proper parent base;
11. traditional print_mode for batch build;
12. proper support for "isabelle build -P Pure";
13. clarified signature: more robust bootstrap base;
14. proper startup for Pure: its use_prelude produces stdout before stderr protocol init;
15. proper context for loading Pure;
16. clarified signature of ML_Process vs. Isabelle_Process: proper support for "isabelle build -P -b";
17. close socket explicitly (idempotent);
18. more robust message_context and shutdown, e.g. after errors in protocol loop (see also 3f02bc5a5a03);
19. simplified code and proofs
20. automated proof