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/parser/gram.y

@@ -2972,11 +2972,11 @@ key_match:  MATCH FULL
 			}
 		| MATCH SIMPLE
 			{
-				$$ = FKCONSTR_MATCH_UNSPECIFIED;
+				$$ = FKCONSTR_MATCH_SIMPLE;
 			}
 		| /*EMPTY*/
 			{
-				$$ = FKCONSTR_MATCH_UNSPECIFIED;
+				$$ = FKCONSTR_MATCH_SIMPLE;
 			}
 		;