On the first connection to a WAL-mode database that was not cleanly shut down

On the first connection to a WAL-mode database that was not cleanly shut down
and contains a left-over -shm file, truncate the -shm file to 3 bytes instead
of to 0 bytes. Avoiding a truncation to 0 means that system monitoring tools
can better detect if a process illegitimately tries to truncate a -shm file.
Such a rogue process might think it is being helpful by cleaning up old files,
but there is a race condition that can cause damage to the database.

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