BUG/MINOR: startup: dump keywords only in worker if started with -W -dKAll

BUG/MINOR: startup: dump keywords only in worker if started with -W -dKAll

If haproxy was started with -W -dK*, after master-worker refactoring, we dump
registered keywords to stdout twice in master and in worker processes. This
information is redundant and output has no longer the right format. So, as the
keyword registration happens very early before the fork, let's dump keywords
only in the worker context, if haproxy was launched with -W.

This does not need to be backported, as related to the latest master-worker
refactoring.