has_column_privilege and has_any_column_privilege SQL functions; fix the
information_schema views that are supposed to pay attention to column
privileges; adjust pg_stats to show stats for any column you have select
privilege on; and fix COPY to allow copying a subset of columns if the user
has suitable per-column privileges for all the columns.

To improve efficiency of some of the information_schema views, extend the
has_xxx_privilege functions to allow inquiring about the OR of a set of
privileges in just one call.  This is just exposing capability that already
existed in the underlying aclcheck routines.

In passing, make the information_schema views report the owner's own
privileges as being grantable, since Postgres assumes this even when the grant
option bit is not set in the ACL.  This is a longstanding oversight.

Also, make the new has_xxx_privilege functions for foreign data objects follow
the same coding conventions used by the older ones.

Stephen Frost and Tom Lane