ccanlint: rework so checks have more structure.

ccanlint: rework so checks have more structure.

Previously each check returned a void *, but in fact most of them fell into
similar patterns.  So define 'struct score' and a helper to add files to it,
and use that.

Under these rules, you get 0/1 if you skip a test because a dependency failed
which in theory means your score (as a percentage) could drop if you fix
a test.