Fix full-table-vacuum request mechanism for MultiXactIds

While autovacuum dutifully launched anti-multixact-wraparound vacuums
when the multixact "age" was reached, the vacuum code was not aware that
it needed to make them be full table vacuums.  As the resulting
partial-table vacuums aren't capable of actually increasing relminmxid,
autovacuum continued to launch anti-wraparound vacuums that didn't have
the intended effect, until age of relfrozenxid caused the vacuum to
finally be a full table one via vacuum_freeze_table_age.

To fix, introduce logic for multixacts similar to that for plain
TransactionIds, using the same GUCs.

Backpatch to 9.3, where permanent MultiXactIds were introduced.

Andres Freund, some cleanup by Álvaro

src/include/access/multixact.h

@@ -87,6 +87,8 @@ extern void MultiXactIdSetOldestMember(void);
 extern int GetMultiXactIdMembers(MultiXactId multi, MultiXactMember **xids,
 					  bool allow_old);
 extern bool MultiXactIdPrecedes(MultiXactId multi1, MultiXactId multi2);
+extern bool MultiXactIdPrecedesOrEquals(MultiXactId multi1,
+							MultiXactId multi2);
 
 extern void AtEOXact_MultiXact(void);
 extern void AtPrepare_MultiXact(void);