oddHeaderMarkup and its kin are \paper variables?

[Mark Polesky]

These are essentially \paper variables, right?
  oddHeaderMarkup
  evenHeaderMarkup
  oddFooterMarkup
  evenFooterMarkup

You have to set them in the \paper block, it seems, so I
would like to categorize them alongside things like
system-separator-markup.  Would that make sense?

So maybe this is something for GLISS, but in keeping with
the naming format of \paper variables, we should probably
change these to:
  odd-header-markup
  even-header-markup
  odd-footer-markup
  even-footer-markup

Thoughts?
- Mark