cmdgen: refuse to generate RSA/DSA keys under 256 bits.

This replicates the existing error message in Windows PuTTYgen, on the
basis that keys smaller than that will actually fail to work with our
signing code.

cmdgen.c

@@ -515,6 +515,14 @@ int main(int argc, char **argv)
         errs = TRUE;
     }
 
+    if (keytype == RSA2 || keytype == RSA1 || keytype == DSA) {
+        if (bits < 256) {
+            fprintf(stderr, "puttygen: cannot generate %s keys shorter than"
+                    " 256 bits\n", (keytype == DSA ? "DSA" : "RSA"));
+            errs = TRUE;
+        }
+    }
+
     if (errs)
 	return 1;