mysqld (initiated by: unknown): Normal shutdown - "unknown" to be specific

From https://mariadb.com/kb/en/how-to-identify-this-mysqld-initiated-by-unknown-normal-shutdown

"unknown" is by a signal rather than a user.

GA - can we change unknown to "signal" (and how would translations work?)

non-GA - can we use sigwaitinfo and its siginfo_t structure to take si_pid, si_uid (anything else interesting?) into the error message.