Fix memory leaks in plane interception

Fix memory leaks in plane interception

Interception builds lists of planes that could intercept.  Only list
nodes for missiles were freed.  Broken since BSD Empire 1.1.

The fix frees interceptors that actually intercepted when
ac_intercept() returns, and the interceptors that didn't when
ac_encounter() returns.

The latter introduces a small bug: it passes planes that didn't fly to
pln_put().  pln_put() expects only planes that actually took off.
Same bug exists in air defense missions.  Luckily, it has no ill
effects.  To be fixed soon.