Add a check on file size as an additional safety check that a WAL file

Add a check on file size as an additional safety check that a WAL file
recovered from archive is not corrupt.  It's not much but it will catch
one common problem, viz out-of-disk-space.
Also, force a WAL recovery scan when recovery.conf is present, even if
pg_control shows a clean shutdown.  This allows recovery with a tar backup
that was taken with the postmaster shut down, as per complaint from
Mark Kirkwood.