xml_merge1(): Allow merging trees with nodes below mountpoints

xml_merge1() fails right now when merging trees which have data
below mountpoints with an error message like this:

XML node %s/%s has no corresponding yang specification (Invalid XML or wrong Yang spec?

This is because xml_merge1() does not handle mountpoints. Adjust
it accordingly.

Note that this was encountered when generating the state of a a yang
module that has both state and config below a mountpoint. In that case
netconf_trymerge() will be called to merge state and config and will
trigger the problem in xml_merge1().

lib/src/clixon_xml_map.c

@@ -1310,6 +1310,22 @@ xml_merge1(cxobj              *x0,  /* the target */
             x1cname = xml_name(x1c);
             /* Get yang spec of the child */
             if ((yc = yang_find_datanode(y0, x1cname)) == NULL){
+                /*
+                 * Actually the CLICON_YANG_SCHEMA_MOUNT option should be checked below
+                 * to be consistent with what is done e.g. in
+                 * clixon_datastore_write.c::text_modify() when yang_find_datanode()
+                 * returns NULL.
+                 * However the clixon_handle needed to check this option is not
+                 * available here.
+                 * So check for the YANG_FLAG_MOUNTPOINT flag on y0 as an alternative.
+                 * It will only have been set if CLICON_YANG_SCHEMA_MOUNT is enabled
+                 * and it will be set for exactly those cases where the xml_spec()
+                 * call is needed.
+                 */
+                if (yang_flag_get(y0, YANG_FLAG_MOUNTPOINT))
+                    yc = xml_spec(x1c);
+            }
+            if (yc == NULL) {
                 if (reason){
                     if ((cbr = cbuf_new()) == NULL){
                         clixon_err(OE_XML, errno, "cbuf_new");