Bug fix: error sorting.

Error sorting can't be done as a postprocess function called from
SyntasticMake(), since the final values of some relevant fields (f.i.
type) might not be known yet at that point.  Solution: move sorting to
getLocListRaw(), after per-checker quiet_messages.  New checker methods
getWantSort() / setWantSort() are needed.

Second problem: some checkers return screen columns mixed with byte
indices.  Solution: compute screen columns as needed.  Sadly, everything
about working with screen columns is fragile.