Widen xl_len field of XLogRecord header to 32 bits, so that we'll have

a more tolerable limit on the number of subtransactions or deleted files
in COMMIT and ABORT records.  Buy back the extra space by eliminating the
xl_xact_prev field, which isn't being used for anything and is rather
unlikely ever to be used for anything.
This does not force initdb, but you do need to do pg_resetxlog if you
want to upgrade an existing 8.0 installation without initdb.