Highlights:
- Rip relative path calculation bits out of xinstall, push it into libutil as relpath(3); link xinstall against libutil
- Use relpath(3) to implement ln -r, create relative symlink (matches similar argument to GNU ln)
- Fix DESTDIR support for certctl(8) using the newly added ln -r
- Finally, fix the caroot package's post-install script to honor PKG_ROOTDIR, which will also now actually work because we're installing relative symlinks
I opted to add -r to ln(1) instead of just using install(1) because it's a useful addition anyways, and a lib'ified relpath(3) is also useful.