BUG #5184: default tablespace owner is not dumped

The following bug has been logged online:

Bug reference:      5184
Logged by:          Robert Haas
Email address:      robertmhaas@gmail.com
PostgreSQL version: CVS HEAD
Operating system:   Linux
Description:        default tablespace owner is not dumped
Details:

The following command does not change the output of "pg_dumpall":

alter tablespace pg_default owner to bob;

But this one does:

alter tablespace foo owner to bob;

The problem is that we only emit CREATE TABLESPACE commands for non-system
tablespaces.  That is correct, but it seems like we might need to emit ALTER
TABLESPACE commands instead for system tablespaces.