add foregin_tables_are functions.

the *_are functions are used by pg_tapgen in the generated
test scripts.
fix foreign_table_are default messages.

add foreign adjective to the default messages.
tests for foreign_tables_are() functions

Replicated the tests used by tables_are.
Add section for foreign_tables_are() functions.

Documentaiton for foreign_tables_are fucntions.
I used the tables_are documentation as a starting point.
Add fake tests for foreign_tables_are functions in older postgres.

Older version of postgres do not support foreign_tables. Rewrite
the tests for the foreign_tables_are functions to check the version
and run the real tests if foreign tables are supported. If an older
version of postgres run pass/fail tests which create the same output
as the real tests.
Use an execute string for newer syntax.

Put the create foreign table, et al into a string to execute so it
can still be parsed by older postgresql versions.
Add foreign_tables_are() to upgrade script.

And credit Chad for his contribution!
Merge branch 'slaught-foreign_tables_are'