elgamal: add privkey_for_block and bytes_per_block
elgamal: use twin-primes-constant approximation for safe primes density
elgamal: default safe prime size: 1024 -> 1025

With a safe-prime of 1024 bits, we would only have 1023 useable bits per
block.  That would not align nicely with the 16 byte blocksize of AES.