Several problems in tab-completions for SET/RESET

Hi,

I found that the following tab-completions for SET/RESET which
worked properly before doesn't work properly now in the master.

1. ALTER SYSTEM SET|RESET <tab> lists nothing.
2. ALTER DATABASE xxx SET <tab> lists nothing.
3. ALTER DATABASE xxx SET yyy <tab> lists nothing.
4. ALTER DATABASE xxx SET datestyle TO <tab> lists nothing.

Attached patch fixes those problems.

Regards,

--
Fujii Masao