Refer to the default foreign key match style as MATCH SIMPLE internally.

Previously we followed the SQL92 wording, "MATCH <unspecified>", but since
SQL99 there's been a less awkward way to refer to the default style.

In addition to the code changes, pg_constraint.confmatchtype now stores
this match style as 's' (SIMPLE) rather than 'u' (UNSPECIFIED).  This
doesn't affect pg_dump or psql because they use pg_get_constraintdef()
to reconstruct foreign key definitions.  But other client-side code might
examine that column directly, so this change will have to be marked as
an incompatibility in the 9.3 release notes.

src/backend/utils/adt/ruleutils.c

@@ -1184,7 +1184,7 @@ pg_get_constraintdef_worker(Oid constraintId, bool fullCommand,
 					case FKCONSTR_MATCH_PARTIAL:
 						string = " MATCH PARTIAL";
 						break;
-					case FKCONSTR_MATCH_UNSPECIFIED:
+					case FKCONSTR_MATCH_SIMPLE:
 						string = "";
 						break;
 					default: