[PATCH] Add a check_hook for output_plugin_libraries

Hi all,

Fujii-san noted in [1] that the new output_plugin_libraries GUC
doesn't have its syntax validated until it's actually used. This is
only a problem if a DBA/superuser makes a mistake, but it would be
really annoying for them to hit it, and it contradicts an assumption I
made inside the new pg_upgrade check. Attached is a patch to fix that.

This isn't ready for backpatching yet, because the new test is added
in a place that doesn't exist in some of the older branches. I'll look
for a better location, but if anyone has a preference off the bat, let
me know.

For PG20 onwards, I think it'd be really nice to enforce that all
GUC_LIST_QUOTE variables must at least pass SplitGUCList() validation
if they don't provide their own explicit check_hook. We have other
code and comments implying that clients don't have to worry about this
case; an example is in [2].

Thanks,
--Jacob

[1] https://postgr.es/m/CAHGQGwG81iHN46mY_19Bne5RuKfAjH4dOnQ7-8i%3D7KYJk_0rAg%40mail.gmail.com
[2] https://postgr.es/m/CAOYmi%2B%3DTomajOOr3OkshSMrNWERAc9fT1je_TEy6Z1UseNy-hA%40mail.gmail.com