Partially revert "options: remove OPT_FLAG"

The m_option_type_flag is still needed for the CONF_TYPE_FLAG
(next commit).

Add a comment that m_option_type_flag shouldn't be used.

ref. #11373

This partially reverts commit 07545657bf.

options/m_option.c

@@ -224,8 +224,71 @@ const m_option_type_t m_option_type_bool = {
 
 #undef VAL
 
+// Flag
+
+#define VAL(x) (*(int *)(x))
+
+static int parse_flag(struct mp_log *log, const m_option_t *opt,
+                      struct bstr name, struct bstr param, void *dst)
+{
+    bool bdst = false;
+    int r = parse_bool(log, opt, name, param, &bdst);
+    if (dst)
+        VAL(dst) = bdst;
+    return r;
+}
+
+static char *print_flag(const m_option_t *opt, const void *val)
+{
+    return print_bool(opt, &(bool){VAL(val)});
+}
+
+static void add_flag(const m_option_t *opt, void *val, double add, bool wrap)
+{
+    bool bval = VAL(val);
+    add_bool(opt, &bval, add, wrap);
+    VAL(val) = bval;
+}
+
+static int flag_set(const m_option_t *opt, void *dst, struct mpv_node *src)
+{
+    bool bdst = false;
+    int r = bool_set(opt, &bdst, src);
+    if (r >= 0)
+        VAL(dst) = bdst;
+    return r;
+}
+
+static int flag_get(const m_option_t *opt, void *ta_parent,
+                    struct mpv_node *dst, void *src)
+{
+    return bool_get(opt, ta_parent, dst, &(bool){VAL(src)});
+}
+
+static bool flag_equal(const m_option_t *opt, void *a, void *b)
+{
+    return VAL(a) == VAL(b);
+}
+
+// Only exists for libmpv interopability and should not be used anywhere.
+const m_option_type_t m_option_type_flag = {
+    // need yes or no in config files
+    .name  = "Flag",
+    .size  = sizeof(int),
+    .flags = M_OPT_TYPE_OPTIONAL_PARAM | M_OPT_TYPE_CHOICE,
+    .parse = parse_flag,
+    .print = print_flag,
+    .copy  = copy_opt,
+    .add   = add_flag,
+    .set   = flag_set,
+    .get   = flag_get,
+    .equal = flag_equal,
+};
+
 // Integer
 
+#undef VAL
+
 static int clamp_longlong(const m_option_t *opt, long long i_min, long long i_max,
                           void *val)
 {