pgsql: Fix unwanted denial of ALTER OWNER rights to superusers.

Log Message:
-----------
Fix unwanted denial of ALTER OWNER rights to superusers. There was some
discussion of getting around this by relaxing the checks made for regular
users, but I'm disinclined to toy with the security model right now,
so just special-case it for superusers where needed.

Modified Files:
--------------
pgsql/src/backend/commands:
aggregatecmds.c (r1.28 -> r1.29)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/aggregatecmds.c.diff?r1=1.28&r2=1.29)
conversioncmds.c (r1.21 -> r1.22)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/conversioncmds.c.diff?r1=1.21&r2=1.22)
dbcommands.c (r1.170 -> r1.171)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/dbcommands.c.diff?r1=1.170&r2=1.171)
functioncmds.c (r1.65 -> r1.66)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/functioncmds.c.diff?r1=1.65&r2=1.66)
opclasscmds.c (r1.35 -> r1.36)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/opclasscmds.c.diff?r1=1.35&r2=1.36)
operatorcmds.c (r1.24 -> r1.25)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/operatorcmds.c.diff?r1=1.24&r2=1.25)
schemacmds.c (r1.33 -> r1.34)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/schemacmds.c.diff?r1=1.33&r2=1.34)
tablecmds.c (r1.166 -> r1.167)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/tablecmds.c.diff?r1=1.166&r2=1.167)
typecmds.c (r1.79 -> r1.80)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/commands/typecmds.c.diff?r1=1.79&r2=1.80)