when split put in a directory

Hi,

When the output is split the current default is to put it in the current
directory. Maybe we could instead put it in a directory named after the manual
name. This is bad as this wouldn't be backward compatible, but
- it is what makeinfo does
- it allows for an unsplit manual in the base directory, while the split 
  manual is in a sub directory, and allows for having a lot of manuals in the
  same directory, as the splitted files are in sub directory.
- it is consistent with what will be the cross manual references produced
  by makeinfo, and texi2html if we follow the same conventions (and I strongly 
  support that, such that we can do ref to manuals generated by makeinfo
  and vice versa).

Pat