plugins ext FEATURE initial schema-mount support

Only for XML data for now. Includes lots of other
changes needed to support this extension.
diff --git a/src/path.c b/src/path.c
index e631e3a..932a039 100644
--- a/src/path.c
+++ b/src/path.c
@@ -446,6 +446,7 @@
             break;
         case LY_VALUE_CANON:
         case LY_VALUE_XML:
+        case LY_VALUE_STR_NS:
             /* not really defined or accepted */
             LOGINT_RET(ctx);
         }