- Add support for Schematron validation

- Add a simple Schematron file that validates file references
- Silence the output of validation so that the result be more readable

share/xml/freebsd.sch

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="iso-8859-1"?>
+
+<!-- $FreeBSD$ -->
+
+<schema xmlns="http://purl.oclc.org/dsdl/schematron">
+  <pattern name="Check file reference validity">
+    <rule context="//*/@fileref">
+      <assert test="contains(., '.')">File reference does not have an extension.</assert>
+    </rule>
+  </pattern>
+</schema>