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.

src/if_perlsfio.c

@@ -51,7 +51,7 @@ sfdcnewvim(void)
 {
     Sfdisc_t	*disc;
 
-    disc = (Sfdisc_t *)alloc(sizeof(Sfdisc_t));
+    disc = ALLOC_ONE(Sfdisc_t);
     if (disc == NULL)
 	return NULL;