Number of cabins is incorrectly reported

[pcercuei]

Observed behaviour

With 3 cabins equipped, one used and two available, the game reports in the <F4> menu:
Cabins: 33% 1t used, 3t free

Expected behaviour

The game should report:
Cabins: 33% 1t used, 2t free

My pioneer version (and OS):
git master (e255171), Debian x64

[impaktor]

Seems like the cabin always shows the total capacity as the current free capacity:
I've filled all my 3 cabins, and the pigui StationView:shipSummary() shows "3t used/3t free".

The NewUI isn't much better, it shows "2 used, 0 free" (should be "3 used, 0 free"). That's a minor point, as we're killing that off today, with #4775.

[impaktor]

Looks like it's just counting the number of cabins in the "cabin" slot, however, both the occupied and empty cabin are in that slot, thus this error.

[pcercuei]

Still not working right. Now I get Cabins: 50% 1 used, 2 free. It should be 33%.