client.c: Fix web UI job queries on Classes/Printers pages

[zdohnal]

Web UI query on Classes/Printers should support:

• search for class/printer (/?QUERY=)
• manage printers (/?)
• listing jobs for the printer/class (/name?Which_jobs=completed)
• show NOT FOUND for non-existing destinations

The PR reworks the code to fix the 3rd point and support all other at the same time.

Fixes Apple/CUPS #6204