Plug memory leaks in mission execution code

Mission execution first builds lists of eligible units, one list per
country.  These lists are passed to perform_mission() one by one,
where they get freed.

Bugs:

* unit_interdict() didn't pass the list for the submarine's owner, but
  build_mission_list_type() built one.  Any submarine movement within
  own submarine interdiction mission op areas leaked memory.

* dosupport() passed only lists for countries that actually support
  (ally at war with victim), but build_mission_list_type() built lists
  for all countries hostile to the victim.  Ground combat within
  support mission op areas countries that are hostile to one of the
  party without actually supporting the other leaked memory.

* perform_mission() failed to free missiles targeting units.

Fixing the latter is straightforward.

Fix the first two by deciding whether a country acts on a mission
trigger before building any lists, in ground_interdict(),
unit_interdict(), dosupport().  Remove the code dealing with that from
build_mission_list_type() and the loops around perform_mission().