Fix some bogosities in pg_dump's foreign-table support.

The server name for a foreign table was not quoted at need, as per report
from Ronan Dunklau.  Also, queries related to FDW options were inadequately
schema-qualified in places where the search path isn't just pg_catalog, and
were inconsistently formatted everywhere, and we didn't always check that
we got the expected number of rows from them.