path-util: Fix path_is_mount_point for parent mount points in symlink mode

When we have a structure like this:

  /bin -> /usr/bin
  /usr is a mount point

Then path_is_mount_point("/bin", AT_SYMLINK_FOLLOW) needs to look at the pair
/usr/bin and /usr, not at the pair / and /usr/bin, as the latter have different
mount IDs. But we only want to consider the base name, not any parent.

Thus we have to resolve the given path first to get the real parent when
allowing symlinks.

Bug: https://github.com/systemd/systemd/issues/61