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.

doc/src/sgml/catalogs.sgml

@@ -2010,7 +2010,7 @@
       <entry>Foreign key match type:
             <literal>f</> = full,
             <literal>p</> = partial,
-            <literal>u</> = simple (unspecified)
+            <literal>s</> = simple
           </entry>
      </row>