Split WWW target in two stages WWW-1 and WWW-2

Split WWW target in two stages WWW-1 and WWW-2

- split local-WWW too in makefiles;

- local-WWW-1 generates Texinfo source, and Info and PDF output;
local-WWW-2 generates HTML output;

- also change WWW-post to a toplevel target, because it needn't recurse
into subdirectories.

This change allows collecting node trees from all Texinfo sources at
the end of local-WWW-1, so they are available to texi2html in
local-WWW-2.