bug#21521: 24.5; pdb default suggested command

[Stefano Pacifico]

Because it does not seem that standard distributions of Python install a `pdb` 
executable script in standard PATH locations. This wiki, for example, 
https://wiki.python.org/moin/EmacsEditor suggests creating the link manually. 

On the other hand, all distributions of Python come with the `pdb.py` module 
available by default, so `python -m pdb` should always work. 

> On Sep 22, 2015, at 3:54 AM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: Stefano Pacifico <github@pacifico.cc>
>> Date: Mon, 21 Sep 2015 22:16:53 -0400
>> 
>> It seems from reading the code in `gud.el` that the system assumes that the 
>> Emacs function/mode (`pdb` in this case) corresponds to the name of the 
>> executable to invoke to perform the debugging, which is not necessarily the 
>> case for `pdb`. 
> 
> Why isn't that the case for 'pdb'?