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.