Fix underquoting issue in DX_DO.

The m4_map macro takes a "quoted list of argument descriptions ...
[which are] in turn a quoted list of quoted elements, suitable for
m4_apply."  This is not what actually is being passed in DX_DO,
which is underquoted resulting in various problems.

Add a test case to expose this issue, and switch DX_DO to use
m4_map_args which is a better fit.

Thanks to Eric Blake.