Add piuparts-report linktarget_by_template information to known_problems.

...by adding a PRIORITY field to all known problem conf files,
capturing the order of entries in linktarget_by_template, and an
EXPLAIN field with the description text.
piuparts-report - add known Problems class list to Section.
piuparts-report - replace linktarget_by_template with Problem class.
piuparts-report - Add a special Problem case for unknown failures.
Add a skip_kpr option to known_problems/detect_well_known_problems.

To support integration with piuparts-report without suffering the
time penalty for kpr generation.
Restore detect_well_known_errors, using the known_problems library.
piuparts-report - skip kpr generation for speed.

tpl processing is still in piuparts-report. This means that it may
be run independently, and reports will still contain issue summaries.
The tpl-generation process is very fast.