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

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.