PG11 changes pg_proc.proisagg to prokind to support stored procedures and

PG11 changes pg_proc.proisagg to prokind to support stored procedures and
window functions, breaking functions, aggregates, and object look-up within
PPA. This commit fixes all three of these areas. In theory we should probably
add some display bits to help users differentiate between these different
types in the UI, but this at least makes the system work again.

Thanks to @decodeJarvis who initially reported the problem and provided a
proposed fix (see PR#24 on the xzilla repo). Since we needed a more
comprehensive fix, I didn't use the code, but his patch was useful for getting
started.