Fix HTML errors in faq.html

* One instance of the </ul> tag should be </li>
* There's a case mismatch in <a href="#anchor"> targets resulting
  in at least Firefox to not find the link target.

Signed-off-by: DHE <git@dehacked.net>
Add entry on how to report bugs to FAQ

Based on Ryan Yao's suggestion in pull request zfsonlinux/zfs#1943.
I also included a small script to dump all the system's stacks in a
thread-aware manner.

Signed-off-by: DHE <git@dehacked.net>