Fix behaviour of : and :: in Scorefile and also make them match slrn documentation

IMPORTANT NOTE:

This makes : and :: in rules behave as specified in the slrn manual.

Specifically, : means all of the rules must match to apply a score, and '::' means any of the rules can match to apply a score.

Prior to this, they were the other way round, and Score:: didn't work at all (though {:: did)

I also

• reformatted the code
• refactored some of this code heavily to make it easier to read.
• re-enabled all but one of the tests under make check in the usenet-utils directory.

Fixes #158 (closed)