Dedicated xml,json,yang and xsl parser utility programs added

Sanity check of stdarg (...) added
Cleanup of error messages.

test/test_xsl.sh

@@ -0,0 +1,12 @@
+#!/bin/bash
+# Test: XSL tests
+PROG=../lib/src/clixon_util_xsl
+
+# include err() and new() functions and creates $dir
+. ./lib.sh
+
+new "xsl test"
+expecteof $PROG 0 "a
+<a><b/></a>" "^0:<a><b/></a>$"
+
+rm -rf $dir