MINOR: config: add a new message directive: .diag

MINOR: config: add a new message directive: .diag

This one works just like .notice/.warning/.alert except that it prints
the message at level "DIAG" only when haproxy runs in diagnostic mode
(-dD). This can be convenient for example to pass a few hints to help
locate certain config parts or to leave messages about certain temporary
workarounds.

Example:

  .diag "WTA/2021-05-07: $.LINE: replace 'redirect' with 'return' after final switch to 2.4"
         http-request redirect location /goaway if ABUSE