An element whose children break its content model now names each stray
child outside the model's alphabet individually, and reports a pure
order/count violation once with the allowed elements as a hint, instead
of a single opaque "do not match its content model". Adds an alphabet
extractor to the content-model matcher. This brings DTD to the same
located standard as the XSD, RELAX NG, and Schematron validators, so all
four schema languages give editor-grade diagnostics.
