Make the type of the waypoint a non-optional part of the constructor.
Every waypoint needs a type, and there's no good default (the previous
default, `TAKEOFF`, is actually unused). All of the target waypoints
were mistakenly being set as `TAKEOFF`, so I've fixed that in the
process.
Also, fix the bug where only the last custom target of a SEAD
objective was being added to the waypoint list because the append was
scoped incorrectly.