Add self-documenting filesystem:
- Doc() option on FsNodeDecl for human-readable descriptions
- /help now generated from spec at runtime, not loaded from file
- Every file/directory in spec.go has documentation
Also fix file-template Each nodes:
- Each without children but with Read/Write creates files, not dirs
- Update validator, listDir, statDir to recognize file templates
- Fix elevate/{reqid} and proc/{pid} to use file-template pattern