pgsql-server: Widen xl_len field of XLogRecord header to 32 bits, so

Log Message:
-----------
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.

Modified Files:
--------------
pgsql-server/src/backend/access/transam:
xlog.c (r1.166 -> r1.167)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xlog.c.diff?r1=1.166&r2=1.167)
pgsql-server/src/bin/pg_resetxlog:
pg_resetxlog.c (r1.23 -> r1.24)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/bin/pg_resetxlog/pg_resetxlog.c.diff?r1=1.23&r2=1.24)
pgsql-server/src/include/access:
xlog.h (r1.57 -> r1.58)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/access/xlog.h.diff?r1=1.57&r2=1.58)
xlog_internal.h (r1.4 -> r1.5)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/access/xlog_internal.h.diff?r1=1.4&r2=1.5)