Rename pg_validatebackup to pg_verifybackup.
Also, use "verify" rather than "validate" to refer to the process
being undertaken here. Per discussion, that is a more appropriate
term.
Discussion: https://www.postgresql.org/message-id/
172c9d9b-1d0a-1b94-1456-
376b1e017322@2ndquadrant.com
Discussion: http://postgr.es/m/CA+TgmobLgMh6p8FmLbj_rv9Uhd7tPrLnAyLgGd2SoSj=qD-bVg@mail.gmail.com