Do not DROP default roles in pg_dumpall -c

When pulling the list of roles to drop, exclude roles whose names
begin with "pg_" (as we do when we are dumping the roles out to
recreate them).

Also add regression tests to cover pg_dumpall -c and this specific
issue.

Noticed by Rushabh Lathia.  Patch by me.

src/bin/pg_dump/pg_dumpall.c

@@ -605,7 +605,13 @@ dropRoles(PGconn *conn)
 	int			i_rolname;
 	int			i;
 
-	if (server_version >= 80100)
+	if (server_version >= 90600)
+		res = executeQuery(conn,
+						   "SELECT rolname "
+						   "FROM pg_authid "
+						   "WHERE rolname !~ '^pg_' "
+						   "ORDER BY 1");
+	else if (server_version >= 80100)
 		res = executeQuery(conn,
 						   "SELECT rolname "
 						   "FROM pg_authid "