dist-docs: Fix bugs in text to HTML conversion.

dist-docs: Fix bugs in text to HTML conversion.

This fixes two bugs.  First, & has a special meaning in the replacement
text for a sed "s" command, so this escapes it.  Second, this code
misprocessed bold or underlined &<>: >^H> would become &gt;^H&gt; which
would display as &gt&gt; in most browers.

Finally, this improves the HTML output so that bold ABC becomes <b>ABC</b>
instead of <b>A</b><b>B</b><b>C</b>.

Reported-by: Nicolas Bouliane <nbouliane@digitalocean.com>
Reported-at: https://twitter.com/nicboul/status/1126959264772259842
Signed-off-by: Ben Pfaff <blp@ovn.org>