parse partition strategy string in gram.y

Hello

I've had this patch sitting in a local branch for way too long.  It's a
trivial thing but for some reason it bothered me: we let the partition 
strategy flow into the backend as a string and only parse it into the
catalog 1-char version quite late.

This patch makes gram.y responsible for parsing it and passing it down
as a value from a new enum, which looks more neat.  Because it's an
enum, some "default:" cases can be removed in a couple of places.  I
also added a new elog() in case the catalog contents becomes broken.

-- 
Álvaro Herrera               48°01'N 7°57'E  —  https://www.EnterpriseDB.com/
"Estoy de acuerdo contigo en que la verdad absoluta no existe...
El problema es que la mentira sí existe y tu estás mintiendo" (G. Lama)