The current tool name is too restrictive and focuses only on verifying
checksums.  As more options to control checksums for an offline cluster
are planned to be added, switch to a more generic name.  Documentation
as well as all past references to the tool are updated.

Author: Michael Paquier
Reviewed-by: Michael Banck, Fabien Coelho, Seigei Kornilov
Discussion: https://postgr.es/m/20181221201616.GD4974@nighthawk.caipicrew.dd-dns.de