Implement a simple, consistent user interface for error handling.

We now have sufficient practical experience to know what we want,
so this is intended to be final:
- provide -Wlevel (warning, error or fatal) to select what you care about
- provide -Wstop to stop after parsing a file with warnings you care about
- provide consistent exit status codes for those warnings you care about
- fully document what warnings, errors and fatal errors mean
- remove all other cruft from the user interface, less is more:
- remove all -f knobs along with the whole -f option
- remove the old -Werror because calling warnings "fatal" is silly
- always finish parsing each file, unless fatal errors prevent that
This commit also includes a couple of related simplifications behind
the scenes regarding error handling.
Feedback and OK  kristaps@;  Joerg Sonnenberger (NetBSD) and
Sascha Wildner (DragonFly BSD) agree with the general direction.

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