mkdir -o split-html-dir

[Karl Berry]

Hi Patrice,

The gendocs.sh script we use to generate GNU manuals (in texinfo/util)
runs a cmd like this to make the split HTML form:

makeinfo --html -o maintain.html maintain.texi

With "maintain.html" not existing, as either a dir or a file.

The past makeinfo created a directory maintain.html in this case. 
tp doesn't.  Is this intentional?  I can't remember if we discussed it.

Thanks,
Karl