BUG/MINOR: trace: automatically start in waiting mode with "start <evt>"

BUG/MINOR: trace: automatically start in waiting mode with "start <evt>"

The doc clearly says that "start <evt>" should leave the trace in pause
mode until the indicated event appears. However it's not what's happening,
the state is not changed until one command uses "now", so it's typically
needed to configure the events with "start <evt>" then enable the waiting
mode using "pause now". This is counter-intuitive and does not match the
doc, so let's fix it so that "start <evt>" switches from stopped to waiting
as long as at least one event is enabled.

This can be backported to all versions.