Limits(strictInternalSubset: true) rejects conditional sections and
parameter-entity references inside markup declarations in the internal
subset, the two feature classes XML 1.0 reserves for the external
subset. DeclSep references and quoted attribute defaults (AttValue)
stay legal; defaults unchanged. The xmltest runner opts in, leaving
case 141 (pre-Fifth-Edition name classes) as its only deviation.
Closes#128
