shared/exit-status: use Bitmap instead of Sets

shared/exit-status: use Bitmap instead of Sets

I opted to embed the Bitmap structure directly in the ExitStatusSet.
This means that memory usage is a bit higher for units which don't define
this setting:

Service changes:
        /* size: 2720, cachelines: 43, members: 73 */
        /* sum members: 2680, holes: 9, sum holes: 39 */
        /* sum bitfield members: 7 bits, bit holes: 1, sum bit holes: 1 bits */
        /* last cacheline: 32 bytes */

        /* size: 2816, cachelines: 44, members: 73 */
        /* sum members: 2776, holes: 9, sum holes: 39 */
        /* sum bitfield members: 7 bits, bit holes: 1, sum bit holes: 1 bits */

But this way the code is simpler and we do less pointer chasing.