Rework (usage of) *SSL random handling (Rich Salz)..

Rework (usage of) *SSL random handling (Rich Salz)..

Add config.h:n_RANDOM_USE_XSSL and ssl_rand_bytes() and use the
*SSL RAND_* machinery if possible for our random numbers.

Do not skip loading of the *SSL entropy file if the PRNG claims to
have successfully initialized itself.

Do not use RAND_seed() but only RAND_add() to feed entropy in the
PRNG, use an entropy value that is a little bit more realistic
than what we have used before, though likely still too high.
Alway feed in.

Always try to write the updated entropy to the *SSL entropy file
if the PRNG claims to have enough entropy.

Introduce n_random_create_buf() and make n_random_create_cp()
a simple wrapper of that.