Re: libpq-fe.h should compile *entirely* standalone

I wrote:

We can easily do better, as attached, but I wonder which other
headers should get the same treatment.

After a bit of further research I propose the attached.  I'm not
sure exactly what subset of ECPG headers is meant to be exposed
to clients, but we can adjust these patterns if new info emerges.

This is actually moving the inclusion-check goalposts quite far,
but HEAD seems to pass cleanly, and again we can always adjust later.
Any objections?			

LGTM

cheers

andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com