MINOR: errors: add ha_diag_notice() to report diag-level notifications

MINOR: errors: add ha_diag_notice() to report diag-level notifications

Right now the only way to report info that is only displayed in diag
mode with -dD is to use ha_diag_warning(). The problem is that this is
then counted as a warning and may result in errors when combined with
-dW, as happens for the CPU topology info:

  $ printf "global\nstats socket /tmp/sock1\n" | ./haproxy -dD -dW -c -f /dev/stdin; echo $?
  [NOTICE]   (10406) : haproxy version is 3.5-dev0-5091ac-35
  [NOTICE]   (10406) : path to executable is ./haproxy
  [DIAG]     (10406) : Created 20 threads split into 2 groups
  [ALERT]    (10406) : Some warnings were found and 'zero-warning' is set. Aborting.
  1

We need another level. This commit introduces ha_diag_notice() which only
emits a notification that doesn't count as a warning. Note that we could
even introduce an info level and revisit various messages so that notice
only reports certain events while info is for anything (like versions
above). That could be a future improvement.