add XSLT linter

[xworld21]

As per subject, I would like to add an XSLT linter, because I am manipulating the stylesheets with XSLT, and diff'ing the changes gets tricky with all the non-semantic differences caused by libxslt.

My idea would be to run the stylesheets through an (almost) identity XSLT transform, then postprocess the output to recover some of the original style:

• convert literal newlines to 
 inside <xsl:text>;
• convert decimal character references to uppercase hexadecimal;
• reinsert literal newlines inside the attribute values spanning several lines (the XML parser changes newlines into spaces, so one must use a hack here, e.g. restore newline + indentation whenever there is a string of consecutive spaces);
• move the attributes of <xsl:stylesheet> and <xsl:output> to separate lines.

This can be done while keeping the stylesheets XML-equivalent to the original ones, down to the text nodes, although this means one cannot reindent tags. Things that are lost because of XML parsing:

• how attributes and namespaces are laid out within each tag, except for <xsl:stylesheet> and <xsl:output> which are done manually from Perl;
• you cannot use two or ore consecutive spaces inside an attribute value except for indentation; consecutive spaces are never needed, though, so that is a reasonable compromise, I hope;
• you cannot use tabs within attributes, those are converted to spaces and cannot be recovered;
• attributes that are intentionally within single quotes for readability, and there aren't any at the moment.

I have made a simple implementation in this branch. The linter croaks if the reformatted output is not equivalent to the original file, and it seems to work well.

How to read the branch: the pre-linter commits are minor adjustments that are supposed to have no consequence (of course, I submitted them as #1489, and they fail a test with TeXLive 2016, so that's not true! I'll investigate); the linter appears in a single commit; the final commit is how the files look like after linting.