Bug#545676: dh_installdocs: option to link documentation directories

Bug#545676: dh_installdocs: option to link documentation directories

As discussed by e-mail, dh_installdocs could do with an option to make
the documentation directory a symlink, to clean up dh-using packages
that otherwise need to use override targets to arrange for dh_link to
run before dh_installdocs.

This turns out to be slightly involved due to the need to handle the
case where you want to do this *and* also install some extra
documentation in the symlink target, while also making a dangling
symlink and avoiding file conflicts in the simple case, so I had to
change dh_installchangelogs as well. I think this is right now; tested
on debconf ('dh_installdocs -Ndebconf-doc --link-doc=debconf;
dh_installdocs -pdebconf-doc') and groff ('dh_installdocs
--link-doc=groff-base').