#247 panic() unsafe as signal handler

[Markus Armbruster]

Signal handler panic() calls functions not safe for
signal handlers.

It also flushes game data to disk. This can overwrite
good state on disk with bad or inconsistent state.

It is not obvious how to best fix this. Most data
updates write through to disk. If all did, the (risky)
flushing in case of a crash would be superflous.