app-text/mathml2: Rewrite of mathml-xml-dtd.

Reimplement everything as part of an update to EAPI=7.  The new package
name seems more consistent, e.g., this naming complements app-text/xhtml11
in the gentoo repo.

Punt all the XHTML-related bits on installation.  I imagine this is
all obsolete anyway (the XHTML+MathML+SVG DTDs seem to have their own
packages separate from the MathML one), was never registered with the
system catalogs and the docbook mathml stuff doesn't appear to need
it anyway.