pgsql: Allow Hot Standby to begin from a shutdown checkpoint.

Log Message:
-----------
Allow Hot Standby to begin from a shutdown checkpoint.

Patch by Simon Riggs & me

Modified Files:
--------------
pgsql/src/backend/access/transam:
twophase.c (r1.59 -> r1.60)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/twophase.c?r1=1.59&r2=1.60)
xlog.c (r1.393 -> r1.394)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.393&r2=1.394)
pgsql/src/include/access:
twophase.h (r1.14 -> r1.15)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/include/access/twophase.h?r1=1.14&r2=1.15)