Fix SCRAM-SHA-1 SASL mechanism

Fix SCRAM-SHA-1 SASL mechanism

The symbol used in sasl-mechanism-alist needs to match the name that
can be required.  Move sasl-make-mechanism call to end of file, to
ensure that it can refer to the specified step functions.

* sasl.el (sasl-mechanism-alist): Refer to sasl-scram-rfc
instead of sasl-scram-sha-1, as the former is the name that can be
required.

* sasl-scram-rfc.el (sasl-scram-sha-1-steps)
(sasl-scram-sha-1-client-final-message)
(sasl-scram-sha-1-authenticate-server): Move to end of file.