SimpleXML schema validate patch

Here is a patch against the latest CVS for SimpleXML to fix its schema
validation support.

It does the following:

1. Includes all the correct libxml2 schema header files.
2. Determines whether schema support is available.
3. Avoids refreeing the parser.
4. Inverts valid / invalid logic to work correctly.

Please apply.

-adam

PS: Let me know if I generated the patch incorrectly.

--
adam trachtenberg
adam@trachtenberg.com