Parallelize documentation builds.

Parallelize documentation builds.

This speeds up building of documentation on multiprocessor
platforms, and is motivated by Texinfo 5.0, which is much slower.
Add a toplevel rule 'make docs' to make all the documentation.
* .bzrignore: Add .dvi, .html, .ps.
* Makefile.in (DVIS, HTMLS, INFOS, PSS, DOCS): New macros.
($(DOCS), docs, vi, html, pdf, ps): New rules.
(info-real): Depend on $(INFOS) rather than doing it sequentially.
(dvi): Depend on $(DVIS) rather than doing it sequentially.
* doc/misc/Makefile.in (html): New rule.