Improve Pydoc interactive browsing (#2001). Patch by Ron Adam.

* A -b option to start an enhanced browsing session.
* Allow -b and -p options to be used together.
* Specifying port 0 will pick an arbitrary unused socket port.
* A new browse() function to start the new server and browser.
* Show Python version information in the header.
* A *Get* field which takes the same input as the help() function.
* A *Search* field which replaces the Tkinter search box.
* Links to *Module Index*, *Topics*, and *Keywords*.
* Improved source file viewing.
* An HTMLDoc.filelink() method.
* The -g option and the gui() and serve() functions are deprecated.