Make some use of anonymous unions [reloptions]

Make some use of anonymous unions [reloptions]

In the spirit of commit 4b7e6c73b0df and following, which see for more
details; it appears to have been quite an uncontroversial C11 feature to
use and it makes the code nicer to read.

This commit changes the relopt_value struct.

Author: Peter Eisentraut <peter@eisentraut.org>
Author: Álvaro Herrera <alvherre@kurilemu.de>
Note: Yes, this was written twice independently.
Discussion: https://postgr.es/m/202601192106.zcdi3yu2gzti@alvherre.pgsql