Fixes to the 'make cover' targets

[Nick Wellnhofer]

 > See branch nwellnhof/make_cover
 >
 > Changes:

Unfortunately, my original changes broke older versions of Devel::Cover, 
so I took a new approach. I added a new experimental make target 
'quickcover_new' that calls a perl script to generate the coverage 
reports. The new target is available in the master branch and shouldn't 
interfere with the other coverage targets.

I got some positive reports from people who couldn't get 'make cover' 
running before, but I could need some testers with Devel::Cover versions 
0.67 or earlier.

Related commit: 
https://github.com/parrot/parrot/commit/9b71f41bad97d38b5cb28ec12992d51d4cb0485e

Nick