shared/install: do not try to resolve symlinks outside of root directory

I linked a file as root, so I had a symlink /root/test.service ← /etc/systemd/system/test.service.
To my surpise, when running test-systemctl-enable, it failed with a cryptic EACCES.
The previous commit made the logs a bit better. Strace shows that we
were trying to follow the symlink without taking --root into account.

It seems that this bug was introduced in 66a19d85a5:
before it, we'd do readlink_malloc(), which returned a path relative to root. But
we only used that path for checking if the path is in remove_symlinks_to set, which
contains relative paths. So if the path was relative, we'd get a false-negative
answer, but we didn't go outside of the root. (We need to canonicalize the symlink
to get a consistent answer.) But after 66a19 we use chase_symlinks(), without taking
root into account which is completely bogus.

src/shared/install.c

@@ -611,7 +611,7 @@ static int remove_marked_symlinks_fd(
                                 return -ENOMEM;
                         path_simplify(p);
 
-                        q = chase_symlinks(p, NULL, CHASE_NONEXISTENT, &dest, NULL);
+                        q = chase_symlinks(p, lp->root_dir, CHASE_NONEXISTENT, &dest, NULL);
                         if (q == -ENOENT)
                                 continue;
                         if (q < 0) {