pgsql: Check for partial WAL files in standby mode.

Log Message:
-----------
Check for partial WAL files in standby mode. If restore_command restores
a partial WAL file, assume it's because the file is just being copied to
the archive and treat it the same as "file not found" in standby mode.
pg_standby has a similar check, so it seems reasonable to have the same
level of protection in the built-in standby mode.

Modified Files:
--------------
    pgsql/src/backend/access/transam:
        xlog.c (r1.370 -> r1.371)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xlog.c?r1=1.370&r2=1.371)