crypto: pbkdf2 deprecate digest overload.

As per #3292, this PR introduces a deprecation notice about removing
the 'default digest' overload which currently defaults to the soon
to be defunct SHA1 digest.

Instead it should be left up to the documentation and implementor to
suggest a suitable digest function.

Ref: https://github.com/nodejs/node/pull/3292
PR-URL: https://github.com/nodejs/node/pull/4047


Reviewed-By: bnoordhuis - Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor@indutny.com>
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>