patch 8.2.0559: clearing a struct is verbose

Problem:    Clearing a struct is verbose.
Solution:   Define and use CLEAR_FIELD() and CLEAR_POINTER().

src/main.c

@@ -119,7 +119,7 @@ main
     // Many variables are in "params" so that we can pass them to invoked
     // functions without a lot of arguments.  "argc" and "argv" are also
     // copied, so that they can be changed.
-    vim_memset(&params, 0, sizeof(params));
+    CLEAR_FIELD(params);
     params.argc = argc;
     params.argv = argv;
     params.want_full_screen = TRUE;