Adds a 'new' warning category.

The 'new' category is not a subcategory of 'all' so we need to make sure that
'use warnings' or 'use warnings qw/all/' doesn't enable the 'new' category.
To achieve this we modify the $^WARNINGS_BITS magic setter. With this patch
the -W flag still means enable *all* warnings. That is 'perl -W' enables 'new'
warnings aswell as 'all' warnings.

What this commit *doesn't* do is change the behaviour of 'no warnings', it only
disables 'all' warnings. If you've enabled 'new' warnings then you need to
'no warnings qw/new/' to disable them.

If you want 'no warnings' to disable *all* warnings both 'new' and 'all' then you
can patch the unimport method of warnings.pm. But before you do that you should
answer the question: Should 'no warnings' and 'no warnings qw/all/' remain equivalent?
'no warnings' and 'no warnings qw/all/' also disables the 'new' category