This mostly involves making it work with the objectaddress.c framework,
which does most of the heavy lifting.  In that vein, change
GetForeignDataWrapperOidByName to get_foreign_data_wrapper_oid and
GetForeignServerOidByName to get_foreign_server_oid, to match the
pattern we use for other object types.

Robert Haas and Shigeru Hanada