Do not require 'public' to exist for pg_dump -c

Commit 330b84d didn't contemplate the case where the public schema
has been dropped and introduced a query which fails when there is no
public schema into pg_dump (when used with -c).

Adjust the query used by pg_dump to handle the case where the public
schema doesn't exist and add tests to check that such a case no longer
fails.

Back-patch the specific fix to 9.6, as the prior commit was.

Adding tests for this case involved adding support to the pg_dump
TAP tests to work with multiple databases, which, while not a large
change, is a bit much to back-patch, so that's only done in master.

Addresses bug #14650
Discussion: https://www.postgresql.org/message-id/20170512181801.1795.47483%40wrigleys.postgresql.org

Author: sfrost

src/bin/pg_dump/pg_dump.c

@@ -3659,9 +3659,14 @@ getNamespaces(Archive *fout, int *numNamespaces)
 		 * essentially a no-op because the new public schema won't have an
 		 * entry in pg_init_privs anyway, as the entry will be removed when
 		 * the public schema is dropped.
+		 *
+		 * Further, we have to handle the case where the public schema does
+		 * not exist at all.
 		 */
 		if (dopt->outputClean)
-			appendPQExpBuffer(query," AND pip.objoid <> 'public'::regnamespace");
+			appendPQExpBuffer(query," AND pip.objoid <> "
+									"coalesce((select oid from pg_namespace "
+									"where nspname = 'public'),0)");
 
 		appendPQExpBuffer(query,") ");