Hi,

I noticed two problems with the mplib `get_...` family of functions:

- First there seems to be a LuaTeX specific documentation bug:
  In the LuaTeX manual these functions are listed as `getboolean`,
  `getnumeric`, `getstring`and `getpath`, but in the actual
  implementation the names contain a underscore.

- If `get_numeric` is called while two nested loops are in effect in MetaPost,
  an endless loop is triggered when MetaPost tries to iterate
  through the current loops.

I attached a patch fixing both problems. The second point also applies to the standalone mplib.