* MODULES.html.sh (seen_modules, seen_files): New variables.

(func_module): Rewrite to use a few less gnulib-tool and sed
invocations.  Avoid a couple of quadratic algorithms for ...
(missed_modules, missed_files): ... these, with ...
(func_append, func_tmpdir): ... these new functions, from
gnulib-tool.  Analogously, install traps for cleanup.