pgsql: Add restartpoint_command option to recovery.conf.

Log Message:
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Add restartpoint_command option to recovery.conf. Fix bug in %r handling
in recovery_end_command, it always came out as 0 because InRedo was
cleared before recovery_end_command was executed. Also, always take
ControlFileLock when reading checkpoint location for %r.

The recovery_end_command bug and the missing locking was present in 8.4
as well, that part of this patch will be backported separately.

Modified Files:
--------------
pgsql/doc/src/sgml:
recovery-config.sgml (r2.2 -> r2.3)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/recovery-config.sgml?r1=2.2&r2=2.3)
pgsql/src/backend/access/transam:
xlog.c (r1.381 -> r1.382)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.381&r2=1.382)