Allow auto_explain to log to NOTICE

This patch allows a user to configure auto_explain to log to NOTICE instead of the server log. This allows automated testing of application-generated queries to ensure correct plans etc from code that can inspect returned notices but not the server log.

I don't see any tests for auto_explain so haven't added any test cases. Happy to do so if that's deemed necessary.

Diff is against master. Also at https://github.com/tomdcc/postgres/tree/auto-explain-notice

Cheers

Tom