M PyShell.py

1. PyShell Rev 1.39, EditorWindow Rev 1.37 fix was not handling a
   multiline prompt.
2. The same fix introduced a bug where hitting <enter> at a previous
   prompt-only line would copy the prompt to the iomark.
3. Move the setting of sys.ps1 earlier, into PyShell.main(), to allow
   this code to work before a shell is started up.
4. If cursor is on the input line in the prompt, and you hit <enter>,
   process the line instead of complaining.
5. If line has no stdin range (this includes the last line before shell
   restart) strip any prompt before recalling.