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.
(Backported from commit d33246700578cddd1cb8ed8164cfbba50aba4ef3)