Deepen DTD processing in the doctype scanner:
- Conditional sections are processed, not skipped: INCLUDE sections have
their declarations scanned, IGNORE sections are discarded, the keyword
may be a parameter-entity reference, and nested sections are handled.
- <!NOTATION> declarations are parsed and stored on DocumentType.
- Unparsed entities (<!ENTITY ... NDATA n>) are parsed and stored with
their notation, distinct from general entities.
- External parameter entities are loaded through the injected resolver so
their declarations expand; the default refusing resolver loads nothing,
keeping XXE closed.
- NOTATION attributes are a distinct type, validated against the declared
notations (value must be listed and the notation declared).
Tracking unparsed entities also lets ENTITY/ENTITIES attribute values be
validated against the declared NDATA entities, closing the gap left when
#73 could only check name syntax.
Closes#72
