Add XML validation

This commit adds a function to check the validity of the icecast.xml
configuration file. The function returns 0 if the file is valid, -1 if
there was an unknown error or error codes as defined in
xmlRelaxNGValidErr.

TODO:
* Add  the schema somewhere into the repository where it makes sense.
* Use the function to validate the configuration file and add handling
for invalid configuration files.

src/cfgfile.c

@@ -25,6 +25,7 @@
 #endif
 #include <libxml/xmlmemory.h>
 #include <libxml/parser.h>
+#include <libxml/relaxng.h>
 
 #include "common/thread/thread.h"
 #include "cfgfile.h"
@@ -626,6 +627,27 @@ int config_initial_parse_file(const char *filename)
     return config_parse_file(filename, &_current_configuration);
 }
 
+/* Has to be called after xmlParseFile(...) */
+int config_validate(const xmlDocPtr doc)
+{
+    int                     status;
+    xmlRelaxNGPtr           schema;
+    xmlRelaxNGValidCtxtPtr  validctxt;
+    xmlRelaxNGParserCtxtPtr rngparser;
+
+    rngparser   = xmlRelaxNGNewParserCtxt("icecast.rng");
+    schema      = xmlRelaxNGParse(rngparser);
+    validctxt   = xmlRelaxNGNewValidCtxt(schema);
+
+    status      = xmlRelaxNGValidateDoc(validctxt, doc);
+
+    xmlRelaxNGFree(schema);
+    xmlRelaxNGFreeValidCtxt(validctxt);
+    xmlRelaxNGFreeParserCtxt(rngparser);
+
+    return status;
+}
+
 int config_parse_file(const char *filename, ice_config_t *configuration)
 {
     xmlDocPtr  doc;