pgsql: Remove hack in pg_tablespace_aclmask() that disallowed

Log Message:
-----------
Remove hack in pg_tablespace_aclmask() that disallowed permissions
on pg_global even to superusers, and replace it with checks in various
other places to complain about invalid uses of pg_global. This ends
up being a bit more code but it allows a more specific error message
to be given, and it un-breaks pg_tablespace_size() on pg_global.
Per discussion.

Modified Files:
--------------
pgsql/src/backend/catalog:
aclchk.c (r1.140 -> r1.141)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/aclchk.c?r1=1.140&r2=1.141)
heap.c (r1.323 -> r1.324)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/heap.c?r1=1.323&r2=1.324)
index.c (r1.285 -> r1.286)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/catalog/index.c?r1=1.285&r2=1.286)
pgsql/src/backend/commands:
dbcommands.c (r1.199 -> r1.200)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c?r1=1.199&r2=1.200)
tablecmds.c (r1.233 -> r1.234)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c?r1=1.233&r2=1.234)