* doc/Makefile.in, doc/Makefile.sub (.ms.html): Add -P-V switch to

* doc/Makefile.in, doc/Makefile.sub (.ms.html): Add -P-V switch to
the production of html files.
(pic.html): Ditto.
Remove background colour switch.
(.SUFFIXES, clean): Handle *.xhtml files.

* src/devices/grohtml/grohtml.man: Document that -V option now
generates an HTML-4.01 validator or XHTML validator.

* src/devices/grohtml/html-text.cpp (html_text::end_tag,
html_text::start_tag): Suppress <small>, <big>, and <color> tags if
we are inside a <pre> block.

* src/devices/grohtml/post-html.cpp
(html_printer::handle_valid_flag): Add a parameter to indicate
whether it needs to generate a paragraph block.
(html_printer::write_navigation): Updated.
(html_printer::do_file_components): Generate an HTML-4.01 blue
validator button also.