dh_kcdsagen: Generate cofactor first.

Reorganize the parameter generation so that we generate the cofactor
%$v = (p - 1)/2 q$% first, on its own, and then run a simultaneous
primality search to find %$q$% and %$p$%.  Because %$q$%-sized primes
are (usually) much more common than %$p$%-sized primes, this makes the
search go considerably faster -- though it seems to print many more
dots.