Don't let automatic supply starve the sector containing the sink

Don't let automatic supply starve the sector containing the sink

Automatic supply always leaves enough food to avoid starvation in
supply sources, except for one case: when drawing supplies from the
sector containing the sink.

This behavior contradicted info supply.  However, do_feed() used to
rely on it (it would have wiped out food without it).  Supply use
there was removed in commit 7da69c92, so we can now fix this.

Affected by this is the automatic food supply of land units in combat,
and the food supply in commands supply, load and lload.  Except supply
is disabled due to bugs in the last two.