Cause standalone backend (including bootstrap case) to read the GUC

Cause standalone backend (including bootstrap case) to read the GUC
config file if it exists.  This was already discussed as being a good
idea, and now seems the cleanest way to deal with initdb-time failures
on machines with small SHMMAX.  (The submitted patches instead modified
initdb.sh to pass the correct sizing parameters, but that would still
leave standalone backends prone to failure later.  An admin who needs
to use a standalone backend has enough trouble already, he shouldn't
have to manually configure its shmem settings...)