portabled: create temp file for unit, not directory

open_tmpfile_linkable is used to create a temporary file in the same
directory as the target, but portabled uses the name of the parent
directory instead of the file it intends to create.
In other words, it creats a tmp for /etc/systemd/system.attached instead
of /etc/systemd/system.attached/foo.service.
It still works because it's later moved in the right place.
But as a side effect, it tries the create the file in the parent directory
which is /etc/systemd, and it case of read-only filesystems it fails.