Changes

Summary

1. clarified signature;
2. clarified signature: more explicit types;
3. support "rsync --chmod --chown" via Rsync.Context;
4. tuned;
5. bash: proper bash_process via SSH; getenv: prefer light-weight ssh.execute;
6. clarified signature: support explicit cwd;
7. support bash via SSH;
8. clarified comments;
9. proper support for remote cwd;
10. clarified context for (remote) bash scripts: export variables are optional, support cwd;
11. more operations for SSH.System: bash_process and bash;
12. unused;
13. clarified signature: prefer symbolic isabelle.Path over physical java.io.File;