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>

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Also includes a small HTML fix for the FAQ as-is. I've pushed it as a separate commit ahead of the new FAQ entry.