bug#32420: 26.1.50; Ibuffer: Detect correctly the buffers running a process

[Tino Calancha <tino.calancha@gmail.com>]

>>> How do you see it reporting a process?  What I see is that it reports
>>> a pseudo-file for it.  Am I missing something?
>> Attached 1 picture.
>> It shows at the bottom the line:
>> 3 buffers            2515909                  1 file, 1 process
>
> Ah, okay.  So, to fix this, isn't it better to see if the current
> buffer is a process-buffer of some process in process-list?
It's a bit tricky code.
When such thing run, the summarizer, I believe the current buffer
is *Ibuffer*; you receive 'strings': they are the rows in
column 'Filename/Process'.

There is no direct way to connect each string with its buffer.  The 
original code make the assumption that if 'string' matches something like
(foobar)
then it corresponds to a buffer running a process.
That assumption is wrong, for instance, in several info buffers.

Because you first create all columns, and the summarizer runs at the end,
I thought is simple to add the process as a text property for those
buffers having a process.  Then you don't need any assumption on 'string': 
you just check whether if 'string' has a property 'ibuffer-process' or 
not.