patch 8.1.1414: alloc() returning "char_u *" causes a lot of type casts

Problem:    Alloc() returning "char_u *" causes a lot of type casts.
Solution:   Have it return "void *". (Mike Williams)  Define ALLOC_ONE() to
            check the simple allocations.
diff --git a/src/gui_gtk_x11.c b/src/gui_gtk_x11.c
index 17ef6ea..a994869 100644
--- a/src/gui_gtk_x11.c
+++ b/src/gui_gtk_x11.c
@@ -429,7 +429,7 @@
      * into gui_argv.  Freed later in gui_mch_init().
      */
     gui_argc = 0;
-    gui_argv = (char **)alloc((*argc + 1) * sizeof(char *));
+    gui_argv = ALLOC_MULT(char *, *argc + 1);
 
     g_return_if_fail(gui_argv != NULL);
 
@@ -2157,10 +2157,10 @@
     char_u  *tmp    = NULL;
     char_u  **array = NULL;
 
-    if (data != NULL && len > 0 && (tmp = (char_u *)alloc(len + 1)) != NULL)
+    if (data != NULL && len > 0 && (tmp = alloc(len + 1)) != NULL)
     {
 	n = count_and_decode_uri_list(tmp, data, len);
-	if (n > 0 && (array = (char_u **)alloc(n * sizeof(char_u *))) != NULL)
+	if (n > 0 && (array = ALLOC_MULT(char_u *, n)) != NULL)
 	    n = filter_uri_list(array, n, tmp);
     }
     vim_free(tmp);
@@ -2512,7 +2512,7 @@
 	    if (i == count)
 	    {
 		/* allocate an Atoms array which is one item longer */
-		new_atoms = (Atom *)alloc((count + 1) * sizeof(Atom));
+		new_atoms = ALLOC_MULT(Atom, count + 1);
 		if (new_atoms != NULL)
 		{
 		    memcpy(new_atoms, existing_atoms, count * sizeof(Atom));