Port to trunk the old commit r14895 of v8.4 (styles for the stdlib documentation)

 This commit r14895 comes apparently itself from commit r12010 in branch v8.2
Port to trunk commit r16062 of v8.4 (Correction des entêtes pour la documentation en ligne)
doc: version number in cover.html + updates in coq.inria.fr style

To be continued someday, those style files are full of redundancies...
doc/stdlib: fix the html charset in header.html and co
doc: improved xhtml compatibility (cover, header,...)
doc/stdlib: fix the xhtml validity of the index-list template
coqdoc: fix a few issues with xhtml validity (backport 1636f7 and 754abf1 from v8.4)

 - For the style of identifiers, coqdoc was using a 'type' attribute of
   tag <span>. But this attribute isn't a legal attribute of tag <span>
   according to the xhtml norm. Instead, I propose to use 'title' for that.
   The coqdoc.css now supports both approaches.

 - The names of inner links (cross references #foo) were
   containing arbitrary characters (in the case of a notation string).
   For instance in Utf8_core : <a name=":type_scope:'∀'_x_'..'_x_','_x">
   Instead, when strange characters are detected, we now hash the
   string via Digest, and use this hexa hash as html label.

 - And some whitespace before />
coqdoc.css: fix a few errors
refman: xhtml validity of the cover page
refman: avoid label names with whitespace (unsupported in html)
refman: for xhtml validity, add 'alt' attributes to img
refman/coqdoc.tex: fix two erroneous \url
refman/Omega.tex: do not advertize Pierre Cregut's email for bug reports
refman: remove ?uri=referer in urls pointing to validator.w3.org

 Unfortunately, these ?uri=referer parameters do not work correctly
 now that coq.inria.fr forces the switch to https before answering
 any document. See: http://validator.w3.org/docs/help.html#faq-referer
 I currently see no workaround for that, apart from generating links
 like ?uri=http://the.real.url/of/my/page, which would be quite painful.
 For now, users interested in checking the validity of our pages
 will have to copy-paste the url they want to check after clicking
 on the validator button.
refman: fix broken urls
refman: switch all source files to utf8

 Putting utf8 everywhere helps the maintainance of the online refman.
 And anyway, this is the way to go. We should also chase and migrate
 the few remaining iso-latin-1 files elsewhere in the sources.