[metapost] Fix formatting glitches.

[Andreas Scherer]

Hello, Luigi,

In order to finish this initial round of revisions for MP.W, I reduced 
https://github.com/ascherer/mplibdir to only four significant commits 
that fix the remaining issues on the surface level.

Ideas for further improvements can be found at 
https://github.com/ascherer/mplibdir/issues -- these will require 
changes at deeper levels of the code. I'll try to leave these as 
exercises for 'the reader'. :o)

Cheers,
Andreas