* rsa_md5.c: Represent a signature as an mpz_t, not a string.

Updated calls of md5 functions.
* rsa_sha1.c: Likewise.

* rsa.c (rsa_prepare_public_key): Renamed function, was
rsa_init_public_key.
(rsa_prepare_private_key): Renamed function, was
rsa_init_private_key.

Rev: src/nettle/rsa.c:1.2
Rev: src/nettle/rsa.h:1.3
Rev: src/nettle/rsa_md5.c:1.2
Rev: src/nettle/rsa_sha1.c:1.2