Added support for specifying a filename for a breakpoint, roughly

Added support for specifying a filename for a breakpoint, roughly
according to an idea by Harri Pasanen (but with different syntax).
This affects the 'break' and 'clear' commands and their help
functions.  Also added a helper method lookupmodule().

Also:

- Try to import readline (important when pdb is used from/as a script).
- Get rid of reference to ancient __privileged__ magic variable.
- Moved all import out of functions to the top.
- When used as a script, check that the script file exists.