Sequences are now based on int8, not int4, arithmetic. SERIAL pseudo-type

Sequences are now based on int8, not int4, arithmetic.  SERIAL pseudo-type
has an alias SERIAL4 and a sister SERIAL8.  SERIAL8 is just the same
except the created column is type int8 not int4.
initdb forced.  Note this also breaks any chance of pg_upgrade from 7.1,
unless we hack up pg_upgrade to drop and recreate sequences.  (Which is
not out of the question, but I don't wanna do it.)