Fix sequence creation to set the t_xmin of a sequence's tuple to

FrozenTransactionId, not the XID of the creating transaction.  Without
this it's possible for a reference to a long-gone CLOG record to occur,
per Christian Meunier's bug report of 10-Jan-02.  Worse, the sequence
tuple would become invisible to SELECTs after 2 billion transactions.

Since the fix is applied during sequence creation it does not help
existing databases, unless you drop and recreate every sequence.
However, we intend to force initdb for 7.2RC1 anyway, to fix a pg_proc
error, so I see no need to do more for this problem.