Make ships and land units spot planes along the flight path

Make ships and land units spot planes along the flight path

Sectors already spotted overflying planes in every sector along the
flight path, but ships and land units did that only in the target
sector, once if you got any ships there, in ac_encounter(), once if
you got any land units there, in ac_encounter(), once for ships firing
flak, in ac_shipflak(), and once for land units firing flak, in
ac_landflak().  Remove all that, and generalize ac_encounter()'s code
for sectors to spot planes to include ships and land units.  Unlike
before, ships and land units don't spot allied planes.