Cap the size of numbers we check for primality

Cap the size of numbers we check for primality

We refuse to generate RSA keys larger than 16k and DH keys larger than 10k.
Primality checking with adversarial input is a DoS vector, so simply don't
do this. Introduce a cap of 32k for numbers we try to test for primality,
which should be more than large enough for use withing a non-toolkit crypto
library. This is one way of mitigating the DH_check()/EVP_PKEY_param_check()
issue.

ok jsing miod