replace IDENTIFY_SYSTEM code in receivelog.c with RunIdentifySystem()

Hi,

I see a couple of improvements to receivelog.c and pg_receivewal.c:

1) ReceiveXlogStream in receivelog.c has a duplicate code to execute
IDENTIFY_SYSTEM replication command on the server which can be
replaced with RunIdentifySystem().
2) bool returning ReceiveXlogStream() in pg_receivewal.c is being used
without type-casting its return return value which might generate a
warning with some compilers. This kind of type-casting is more common
in other places in the postgres code base.

Attaching a patch to fix the above. Thoughts?

Regards,
Bharath Rupireddy.