Fix loopholes that let you load and unload foreign commodities

Fix loopholes that let you load and unload foreign commodities

The Chainsaw 3 feature to let you load/unload to a specific amount
through a negative amount argument created loopholes: it let you load
your ships in friendly sectors with the unload command, and unload
friendly ships with the load command.  Likewise for land units, with
allied instead of friendly, of course.

Empire 4.0.0 fixed that for the case of loading a land unit from an
allied sector.  Get rid of that check, and fix it for good in
load_comm_ok().