Append .html suffix to temporary files enabling browsers to recognise it

Append .html suffix to temporary files enabling browsers to recognise it

Occasionally one might read a manual page in a webbrowser, e.g.
"MANPAGER=firefox man -T html jq", however temporary files created for
pagers lack file extensions and most web browsers are unable to detect a
file's content without it.

Special case mandoc(1)'s HTML output format by appending the ".html" suffix
to file names such that browsers will actually render HTML as such instead
of showing it as plain text.

Input schwarze