[Rd] Generated HTML tags not all lowercase

[Paul Roebuck]

Noticed while fixing problem in my package documentation
that some Rd code referencing an environment variable
as \env{PATH} produced the HTML as <CODE>PATH</CODE>.

"<R-2.2.1>/share/perl/R/Rdconv.pm#785-800":
shows there are others as well.

Although R seems to currently target HTML 4.0 (or older)
and not the stricter XHTML standard, converting the
remaining HTML tags to lowercase would be
backward-compatible with the current standard.

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SIGSIG -- signature too long (core dumped)