Fixed 'xml:lang' handling in build scripts

Our build scripts were stripping the 'xml:' prefix from 'xml:lang' because
the 'xml' namespace mapping isn't in the same table as all the other prefixes
in the parser. I've just checked in the following changes to fix this, so
   - 'xml:' is preserved in XML output (so 'xml:lang' remains 'xml:lang')
   - 'xml:' is dropped from HTML output (so 'xml:lang' becomes 'lang')

If you have any comments on how this should be done differently or better,
let me know.

~fantasai