Improve the HINT message of the ALTER command for postgres_fdw

Hi

When 'ALTER FOREIGN DATA WRAPPER OPTIONS' is executed against
postgres_fdw, the HINT message is printed as shown below, even though
there are no valid options in this context.

=# ALTER FOREIGN DATA WRAPPER postgres_fdw OPTIONS (format 'csv');
ERROR: invalid option "format"
HINT: Valid options in this context are:

I made a patch for this problem.

regards,
Kosei Masumura