#80 Capital sector avail - ditch BTUs

[Jeff Cotter]

I keep some civs in my capital between updates to make
BTUs. If I keep them there during update they make
avail only, so I move them right before update, then
back again.

Extra micro management and a silly representation of
reality.

I suggest that commands decrease c-sector's avail and
scrapping existing BTU system.

When c-sector avail goes to zero, no more commands,
well, just the free ones then.

Bombing capital hurts by squeezing avail and hence
player's ability to govern his domain.

Players can keep additional c-sectors as a back up for
busy periods. Just switch capital.

Besides, the acronym 'BTU' is already taken.