runtime(man): improve :Man completion for man-db

runtime(man): improve :Man completion for man-db

On man-db systems, complete with actual man sections and pages, instead
of shell commands.

I tried to come up with a portable solution for multiple man
implementations in https://github.com/vim/vim/discussions/16794 but I
think the differences between implementations were too large to do that
without overly complicated code. So instead, I implemented it for man-db
(which I think is common on Linux) and hopefully left it easier for
other people to implement it on other systems in the future if they want
to.

closes: #16843

Signed-off-by: David Mandelberg <david@mandelberg.org>
Signed-off-by: Christian Brabandt <cb@256bit.org>