core: pahole optimization of struct Unit

We had a lone 'bool job_running_timeout_set:1', which generated a hole. Let's
move things around a bit. The structure is a tiny bit smaller and has less
holes:
        /* size: 1192, cachelines: 19, members: 149 */
        /* sum members: 1175, holes: 3, sum holes: 11 */
        /* sum bitfield members: 27 bits, bit holes: 1, sum bit holes: 7 bits */
        /* bit_padding: 14 bits */
        /* last cacheline: 40 bytes */

        /* size: 1184, cachelines: 19, members: 149 */
        /* sum members: 1175, holes: 1, sum holes: 4 */
        /* sum bitfield members: 27 bits (3 bytes) */
        /* bit_padding: 13 bits */
        /* last cacheline: 32 bytes */