[tex-live] Recommendations for viewer_md

[Karl Berry]

    >   texdoc warning: No viewer_md defined, using viewer_txt.

I tweaked texmf-dist/scripts/texdoc/config.tlu to set up viewer_md
from the usual myriad envvars if set, with a final default of $PAGER:
  MDVIEWER_texdoc, TEXDOCVIEW_md, TEXDOC_VIEWER_MD, PAGER

I don't think any real viewer of md files is usually installed.
Didn't try to detect one.

Doc update will have to wait for another time, or forever :(. --thanks, k.