Import tex-xmltex from pkgsrc-wip.

Import tex-xmltex from pkgsrc-wip.

xmltex implements a non-validating parser for documents matching the
W3C XML Namespaces Recommendation.  The system may just be used to
parse the file (expanding entity references and normalising namespace
declarations) in which case it records a trace of the parse on the
terminal.  Normally however the information from the parse is used to
trigger TeX typesetting code.  Declarations (in TeX syntax) are
provided as part of xmltex to associate TeX code with the start and
end of each XML element, attributes, processing instructions, and with
unicode character data.