We were sometimes ending printer enumeration prematurely,
and the code was confused about the meaning of found_printer.

The new setup follows these rules:
- We *only* end the search prematurely if found_printer
  is set, which indicates that we found the right printer
- We *always* call find_printer_idle exactly once, and
  make it return less than perfect matches like the
  default printer, or the first printer we found

Fixes: #4439