Rename "scram" to "scram-sha-256" in pg_hba.conf and password_encryption.

Per discussion, plain "scram" is confusing because we actually implement
SCRAM-SHA-256 rather than the original SCRAM that uses SHA-1 as the hash
algorithm. If we add support for SCRAM-SHA-512 or some other mechanism in
the SCRAM family in the future, that would become even more confusing.

Most of the internal files and functions still use just "scram" as a
shorthand for SCRMA-SHA-256, but I did change PASSWORD_TYPE_SCRAM to
PASSWORD_TYPE_SCRAM_SHA_256, as that could potentially be used by 3rd
party extensions that hook into the password-check hook.

Michael Paquier did this in an earlier version of the SCRAM patch set
already, but I didn't include that in the version that was committed.

Discussion: https://www.postgresql.org/message-id/fde71ff1-5858-90c8-99a9-1c2427e7bafb@iki.fi