Reject 128-byte keys that are not 1024-bit

Reject 128-byte keys that are not 1024-bit

When we added the check for key size, we required that the keys be
128 bytes.  But RSA_size (which defers to BN_num_bytes) will return
128 for keys of length 1017..1024.  This patch adds a new
crypto_pk_num_bits() that returns the actual number of significant
bits in the modulus, and uses that to enforce key sizes.

Also, credit the original bug3318 in the changes file.