Re: tab completion of enum values is broken

I wrote:
> Oh ... experimenting on macOS (with the system-provided libedit)
> shows no bug here. So I guess we'll need to make this conditional
> somehow, perhaps on USE_FILENAME_QUOTING_FUNCTIONS. That's another
> reason for not going overboard.

After further fooling with that, I concluded that the only workable
solution is a run-time check for whether the readline library included
the leading quote in what it hands us. A big advantage of doing it this
way is that it mostly fixes my complaint #1: by crafting the check
properly, we will include quotes if the user hits TAB without having typed
anything, and we won't complete an incorrectly non-quoted identifier.
There's still nothing to be done about single quotes inside an enum
label, but I'm okay with blowing that case off.

So I think the attached is committable. I've tried it on readline
7.0 (RHEL8) as well as whatever libedit Apple is currently shipping.

regards, tom lane