Change the digest string from "md" to "digest"

Change the digest string from "md" to "digest"

Conform to other modules which were changed at the last minute and this
discrepancy was not noticed.
Retain "md" as an alias so not to break 3rd party backports/tests scripts.

Signed-off-by: Simo Sorce <simo@redhat.com>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/8783)