Need to serialize parent in LevelActionThinker
Support for destination map in exit level funcs

G_ExitLevel and G_SecretExitLevel can take an optional destmap parameter
as in Strife.
Support for tagged exit lines in GMI and LI

Missions can set MI_ALLOWEXITTAG and/or MI_ALLOWSECRETTAG to enable
behavior by default. EMAPINFO can set allowexittags and allowsecrettags
to enable it per-map.
Ability to specify PSX mission for PKE files
Reformatting only
Implement line exit tag capability in ev_actions