pgsql: Relax permissions checks on dbsize functions, per discussion.

Log Message:
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Relax permissions checks on dbsize functions, per discussion. Revert out all
checks for individual-table-size functions, since anyone in the database could
get approximate values from pg_class.relpages anyway. Allow database-size to
users with CONNECT privilege for the target database (note that this is
granted by default). Allow tablespace-size if the user has CREATE privilege
on the tablespace (which is *not* granted by default), or if the tablespace is
the default tablespace for the current database (since we treat that as
implicitly allowing use of the tablespace).

Modified Files:
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pgsql/src/backend/utils/adt:
dbsize.c (r1.13 -> r1.14)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/adt/dbsize.c?r1=1.13&r2=1.14)