Fix user-after-free with `xmllint --xinclude --dropdtd`
The --dropdtd option can leave dangling pointers in entity reference nodes. Make sure to skip these nodes when processing XIncludes. This also avoids scanning entity declarations and even modifying them inadvertently during XInclude processing. Move from a block list to an allow list approach to avoid descending into other node types that can't contain elements. Fixes #237.