code cleanup: we prefer 'CURLcode result'

... for the local variable name in functions holding the return
code. Using the same name universally makes code easier to read and
follow.

Also, unify code for checking for CURLcode errors with:

 if(result) or if(!result)

instead of

 if(result == CURLE_OK), if(CURLE_OK == result) or if(result != CURLE_OK)

lib/strerror.c

@@ -330,7 +330,7 @@ curl_easy_strerror(CURLcode error)
    */
   return "Unknown error";
 #else
-  if(error == CURLE_OK)
+  if(!error)
     return "No error";
   else
     return "Error";
@@ -597,7 +597,7 @@ get_winsock_error (int err, char *buf, size_t len)
     return NULL;
   }
 #else
-  if(err == CURLE_OK)
+  if(!err)
     return NULL;
   else
     p = "error";