Fix use of term "verifier"

Fix use of term "verifier"

Within the context of SCRAM, "verifier" has a specific meaning in the
protocol, per RFCs.  The existing code used "verifier" differently, to
mean whatever is or would be stored in pg_auth.rolpassword.

Fix this by using the term "secret" for this, following RFC 5803.

Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://www.postgresql.org/message-id/flat/be397b06-6e4b-ba71-c7fb-54cae84a7e18%402ndquadrant.com