using manual_html as output directory for HTML

[Patrice Dumas]

Hello,

I think that it is not a good thing to have the base manual name, or the
base manual name with .html prepended as default directory for html
output, for three reasons:
* I think that a directory with the manual name should be reserved for
  other purposes, for example keeping in it include texinfo files,
  images, init files, CSS files, translations...
* it does not generalize well to other output formats.

I propose instead to prepend _html to the base manual name, and use
similar conventions for other (future) split formats.  Currently for
epub, which is a bit special as it is not the final output, which is a
.epub file, the directory name has _epub_package prepended.

The HTML Xref specification would be modified accordingly.

-- 
Pat