Fix gcc sprintf warnings

Gcc 9 has become more vocal with sprintf buffer overflow
checking.  Remove these sprintf warnings by changing the
offending calls to a snprintf wrapper that will explicitly
check the result.

src/spell.c

@@ -715,7 +715,8 @@ int *spell_no;
             Sprintf(lets, "a-zA-%c", 'A' + nspells - 27);
 
         for (;;) {
-            Sprintf(qbuf, "Cast which spell? [%s *?]", lets);
+            Snprintf(qbuf, sizeof(qbuf), "Cast which spell? [%s *?]",
+                     lets);
             ilet = yn_function(qbuf, (char *) 0, '\0');
             if (ilet == '*' || ilet == '?')
                 break; /* use menu mode */