[xml] xmlShell usage on non-stdout

["Vakoc, Mark" <Mark_Vakoc jdedwards com>]

The attached patch allows xmlShell to operate on an output other than
stdout.  Looks like this is what was intended, but never implemented.  I
also made stdout the default if NULL is passed into xmlShell's output
parameter.

Please Note:  this requires changing two public APIs to add the
xmlShellCtxtPtr parameter:  xmlShellPrintNode and xmlShellPrintXPathResult

 <<shell.diff>>

Attachment: shell.diff
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