Calling xsl:original() with keywords

[michaelhkay]

We need to define what happens if xsl:original() is called with keyword arguments.

The answer isn't trivial, because (for 3.0 compatibility reasons) an overriding function isn't required to use the same parameter keywords as the function it overrides.

Perhaps we should recognize that there are functions that do not support argument keywords. This might be the case, for example, with Java or C# extension functions.