gpg: Default to SHA-256 for all signature types on RSA keys.
* g10/main.h (DEFAULT_DIGEST_ALGO): Use SHA256 in --gnupg and SHA1 in strict RFC or PGP modes. * g10/sign.c (make_keysig_packet): Use DEFAULT_DIGEST_ALGO also for RSA key signatures. * configure.ac: Do not allow to disable sha256.