Rename XSpec's $debug to $x:debug

[GoogleCodeExporter]

See comment currently at 
https://code.google.com/p/xspec/source/browse/trunk/src/compiler/generate-tests-
utils.xsl#27:

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'debug' is a common, and common-sense, variable name that can conflict with a 
'debug' parameter or variable in the stylesheet under test (as happened to me). 
 Perhaps this $debug should be $x:debug to avoid conflicts?
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(Anybody know a better way to make a link to particular lines in a particular 
revision of a file that's in SVN?)

The variable isn't used anyway because you currently can't use variables in 
@use-when attributes.

Original issue reported on code.google.com by dev.xspec@menteithconsulting.com on 5 Nov 2012 at 1:09