Replace 'cover' make targets with 'cover_new' targets

The 'cover_new' and 'fullcover_new' targets use a Perl program to run the code
coverage tools appropriately.  This change removes the old 'cover' targets and
replaces them with the 'cover_new' targets.  The `make_cover.pl` program also
now uses the configured values for the code coverage tools as opposed to hard
coded values.

Comments as usual welcome.