bug#1779: 23.0.60; proced with variable-pitch header line

[Juri Linkov]

>>> Certainly the new behavior is untenable; please revert my patch.
>> I've now reverted the patch and reopened this report.
>
> I fixed the patch so it also works correctly when
> scrolling horizontally.

As discussed in
https://lists.gnu.org/archive/html/emacs-devel/2020-12/msg00084.html
I tried to fix this bug in the emacs-28 branch by copying code
from ‘tabulated-list-col-sort’ to ‘proced-sort-header’.

Maybe now variable-pitch could be enabled on proced-head-line.