[PATCH] Add PQconninfoParseParams and PQconninfodefaultsMerge to libpq

This patch came up from discussion about pg_isready.

PQconninfoParseParams is similar to PQconninfoParse but takes two
arrays like PQconnectdbParams. It essentially exposes
conninfo_array_parse().

PQconninfodefaultsMerge essentially exposes conninfo_add_defaults().
It allows you to pass a PQconninfoOption struct and it adds defaults
for all NULL values.

There are no docs yet. I assumed I would let bikeshedding ensue, and
also debate on whether we even want these first.