[KProcessList] Optimize KProcessList::processInfo

Summary:
Optimize KProcessList::processInfo on unix so that it doesn't iterate over all
processes and then filter the list to the requested process. Instead refactor
the code that fetches process info from a single process and use that function.

Test Plan:
Unit Test

BUG: 410945

Reviewers: davidedmundson, broulik, mpyne

Reviewed By: mpyne

Subscribers: mpyne, apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D23205