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/ex_docmd.c b/src/ex_docmd.c
index 2b16a1e..82f9462 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -6550,7 +6550,7 @@
     void
 alist_new(void)
 {
-    curwin->w_alist = (alist_T *)alloc(sizeof(alist_T));
+    curwin->w_alist = ALLOC_ONE(alist_T);
     if (curwin->w_alist == NULL)
     {
 	curwin->w_alist = &global_alist;
@@ -6584,7 +6584,7 @@
      * expansion.  Also, the vimrc file isn't read yet, thus the user
      * can't set the options. */
     p_su = empty_option;
-    old_arg_files = (char_u **)alloc(sizeof(char_u *) * GARGCOUNT);
+    old_arg_files = ALLOC_MULT(char_u *, GARGCOUNT);
     if (old_arg_files != NULL)
     {
 	for (i = 0; i < GARGCOUNT; ++i)