Use an enum class to prevent implicit conversion between trigger
behavior flags and other integers. Fix up various resulting breaks,
which look like bugs:
- Descent 2 editor mode could modify trigger::flags, but used
TRIGGER_FLAG_* values, which specify the actions for a Descent 1
trigger when it executes, not the behavior properties for a trigger.
- Adding a trigger set its flags to 0, then cleared all flags except
TRIGGER_ON. Since the flags were just set to 0, the mask operation is
useless. Remove it.
- trigger_turn_all_ON cleared all flags except TRIGGER_ON. This seems
to be completely wrong. Change it to remove
trigger_behavior_flags::disabled. Descent 1 has no (working) support
for disabling triggers, so make trigger_turn_all_ON exclusive to
Descent 2.
- wall_restore_all would enable TRIGGER_ON in both games. Descent 1
never reads TRIGGER_ON. Descent 2 uses this field for trigger
behavior flags, and TRIGGER_ON is not a behavior flag.
- For Descent 1 builds, remove the modification of the field.
- For Descent 2 builds, change it to clear
trigger_behavior_flags::disabled.