Remove 'keystr' parameter in get_rsa_ssh1_pub.

This parameter returned a substring of the input, which was used for
two purposes. Firstly, it was used to hash the host and server keys
during the initial SSH-1 key setup phase; secondly, it was used to
check the keys in Pageant against the public key blob of a key
specified on the command line.

Unfortunately, those two purposes didn't agree! The first one needs
just the bare key modulus bytes (without even the SSH-1 mpint length
header); the second needs the entire key blob. So, actually, it seems
to have never worked in SSH-1 to say 'putty -i keyfile' and have PuTTY
find that key in Pageant and not have to ask for the passphrase to
decrypt the version on disk.

Fixed by removing that parameter completely, which simplifies all the
_other_ call sites, and replacing it by custom code in those two
places that each does the actually right thing.

cmdgen.c

@@ -813,7 +813,7 @@ int main(int argc, char **argv)
 		ret = rsa_ssh1_loadpub(infilename, BinarySink_UPCAST(blob),
                                        &origcomment, &error);
                 BinarySource_BARE_INIT(src, blob->u, blob->len);
-                get_rsa_ssh1_pub(src, ssh1key, NULL, RSA_SSH1_EXPONENT_FIRST);
+                get_rsa_ssh1_pub(src, ssh1key, RSA_SSH1_EXPONENT_FIRST);
                 strbuf_free(blob);
 
 		ssh1key->comment = dupstr(origcomment);