lib: libyang2 workarounds, argue about keeping it

The purpose of this specific commit is to record the specific
commits related to some workarounds. It is safe to drop
this code once libyang3 will be the official one. Currently,
let's keep both code: the legacy and the new one in order to keep
compiling FRR with Linux distros that are using some old libyang
versions.

Signed-off-by: Vincent Jardin <vjardin@free.fr>

lib/yang.c

@@ -1336,11 +1336,16 @@ LY_ERR yang_lyd_trim_xpath(struct lyd_node **root, const char *xpath)
 
 /*
  * Safe to remove after libyang v2.1.128 is required
+ * XXX it is not removed for libyang3 migration in order
+ * to have a double support of libyang2 and libyang3 support
+ * during an interim period.
  */
 const char *yang_ly_strerrcode(LY_ERR err)
 {
 #ifdef HAVE_LY_STRERRCODE
 	return ly_strerrcode(err);
+#elif defined(HAVE_LY_STRERR)
+	return ly_strerr(err);
 #else
 	switch (err) {
 	case LY_SUCCESS: