add note about stock lua an luajit 5.2 to the manpage

closes #177
drop ./ from some sysdig calls in the manpage
fix manpage formating by using proper markdown syntax
it's sysdig, not Sysdig
install a rebuilt version of the manpage if pandoc is available

sysdig.8 is generated from sysdig.md. When a packager wants to modify
the manpage, he has to modify the manpage and not the source, which is
wrong. Let's rebuild the manpage if pandoc is available.

In a perfect world, we would get rid of the prebuilt version at all,
but pandoc may not be available on some tagets.
Merge pull request #195 from evgeni/man-improvements

manpage improvements
Merge pull request #196 from evgeni/rebuild-man

install a rebuilt version of the manpage if pandoc is available