New trigger option of pg_standby

Hi,

Current pg_standby is dangerous because the presence of the trigger
file causes recovery to end whether or not the next WAL file is available.
So, some *available* transactions may be lost at failover. Such danger
will become high if the standby server has not caught up with the primary.

Attached patch fixes the above problem by adding a new trigger option
to pg_standby; the presence of this new trigger file causes recovery to
end after replaying all the available WAL files. Specifically, pg_standby
acts like 'cp' or 'ln' command while this new trigger file exists.

I've not changed any existing features, so backward-compatibility is
maintained.

Thought?

--
Fujii Masao
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center