Simplify the recommended alternatives to rand()

The CPAN Security Group (CPANSec) is currently working on guides to
generating security-quality random data.  We are focusing on modules
that have secure defaults and are fairly lightweight.

We would like to change the recommended modules to ones that we think
are better options.

Crypt::URandom is pure-perl, has fewer prerequisites than Crypt::Random,
and works with Windows.

Crypt::PRNG has secure defaults and methods for generating different
kinds of random data.

pod/perlfunc.pod

@@ -6669,13 +6669,9 @@ including:
 
 =over
 
-=item * L<Data::Entropy>
+=item * L<Crypt::URandom>
 
-=item * L<Crypt::Random>
-
-=item * L<Math::Random::Secure>
-
-=item * L<Math::TrulyRandom>
+=item * L<Crypt::PRNG>
 
 =back