Refactor permissions checks for large objects.

Refactor permissions checks for large objects.

Up to now, ACL checks for large objects happened at the level of
the SQL-callable functions, which led to CVE-2017-7548 because of a
missing check.  Push them down to be enforced in inv_api.c as much
as possible, in hopes of preventing future bugs.  This does have the
effect of moving read and write permission errors to happen at lo_open
time not loread or lowrite time, but that seems acceptable.

Michael Paquier and Tom Lane

Discussion: https://postgr.es/m/CAB7nPqRHmNOYbETnc_2EjsuzSM00Z+BWKv9sy6tnvSd5gWT_JA@mail.gmail.com