Disable new connections in case of fatal signal

Ticket created for https://github.com/MariaDB/server/commit/a1d51c33611f40d2ede4b36208bd489dbde183cc

Commit Message:
A user reported that MariaDB server got a signal 6 but still accepted new
connections and did not crash. I have not been able to find a way to
repeat this or find the cause of issue. However to make it easier to
notice that the server is unstable, I added code to disable new
connections when the handle_fatal_signal() handler has been called.