dumplog message history groundwork
Separate the message logging out of pline so that other things (for
instance, one-line summary for quest block messages) can be logged.
The code that utilizes this isn't ready for prime time yet.
For FREE_ALL_MEMORY, release DUMPLOG message history when saving.
(Actually, this frees it unconditionally rather just doing so for
FREE_ALL_MEMORY.) It was being freed when logged at end of game,
but not during save. If dumplog message history and interface
message history get integrated, the existing message history
save/restore handling should become applicable instead.