Generate better file name for Tapset html

Generate better file name for Tapset html

As a default xmlto uses reXXXX.html for file names for the generated html files.
This has two drawbacks: it is not meaningful to humans and is likely to change
between builds of the tapset reference manual. The added XMLTOHTMLPARAMS
option generate more nmemonic names for the files and generates files that
are less likely to change between builds.