GUC for temporarily disabling event triggers

In the thread discussing the login event trigger patch it was argued that we
want to avoid recommending single-user mode for troubleshooting tasks, and a
GUC for temporarily disabling event triggers was proposed.

Since the login event trigger patch lost momentum, I've broken out the GUC part
into a separate patch to see if there is interest in that part alone, to chip
away at situations requiring single-user mode.

The patch adds a new GUC, ignore_event_trigger with two option values, 'all'
and 'none' (the login event patch had 'login' as well). This can easily be
expanded to have the different types of events, or pared down to a boolean
on/off. I think it makes more sense to make it more fine-grained but I think
there is merit in either direction.

If there is interest in this I'm happy to pursue a polished version of this
patch.

--
Daniel Gustafsson https://vmware.com/