Fix and modernize contents of head (according to vnu checker)

[cspiel]

The vnu checker as of version 20.6.30 always turns up the following
problems with Hevea-generated HTML files (edited for clarity):

Line 4: error: Internal encoding declaration "us-ascii" disagrees
               with the actual encoding of the document ("utf-8").
Line 4: error: Bad value "text/html; charset=US-ASCII" for
               attribute "content" on element "meta": "charset="
               must be followed by "utf-8".
Line 6: info warning: The "type" attribute for the "style"
                      element is not needed and should be omitted.

This P/R addresses all three diagnostics.

In the course of fixing the problems it turned out that a new
(internal) macro \@print@endline would be beneficial for
the clarity and simplicity of expression. So the P/R has been
split into three patches:

1. Introduce and document \@print@endline.
2. Use the new macro in every position where \@print
   needs to add a final newline character.
3. Fix macro \@meta which generates the relevant part
   of the head-element's contents. Update intertwined lexers
   accordingly.

With this P/R Hevea generates vnu-clean headers.

[maranget]

Hi thanks again for your contribution.

Did your check the impact of this PR on changing the document input (and output) charset, as documented here in the user manual?

[cspiel]

@maranget: Sure, I have checked that this change does not break
output encodings. 😉

Following the link you gave me, I found that the documentation of
package inputenc.hva would benefit of a comprehensive table that
lists all available encodings, their character sets and their relations
to LaTeX's inputenc.sty. Done in 2824a6b.