Expose Bro's linear congruence PRNG as utility function.

It was previously not possible to crank the wheel on the PRNG in a
deterministic way without affecting the globally unique seed. The new extra
utility function bro_prng takes a state in the form of a long int and returns
the new PRNG state, now allowing arbitrary code parts to use the random number
functionality.

This commit also fixes a problem in the H3 constructor, which requires use
of multiple seeds. The single seed passed in now serves as seed to crank out as
many value needed using bro_prng.