debian/panic-action: steer users towards configuration and logs

In https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1089853, a misconfiuration was mistaken for a segmentation fault, even after the reporter found the panic could be avoided by fixing the configuration.

This is likely because the message mentions "segfault" and produces only a stacktrace as diagnostic evidence, which is enough to fool anyone.

The smb_panic() call itself knows more, because it is given a message (in this case "open_sockets_smbd() failed", which may not help), and this is logged, as usually are a number of preceding related messages.

We could with some effort allow the panic action to take a message in $2, but this will be a bit fiddly as we would need to move away from using system() to avoid shell escaping trouble.