(rsa_check_size): New function, for computing and checking

the size of the modulo in octets.
(rsa_prepare_public_key): Usa rsa_check_size.
(rsa_init_private_key): Removed code handling n, e and d.
(rsa_clear_private_key): Likewise.
(rsa_compute_root): Always use CRT.

Rev: src/nettle/rsa.c:1.4