src: rename CryptoPemCallback -> PasswordCallback

While reading through node_crypto.cc I think the code could perhaps
be be a made a little clearer if CryptPemCallback was renamed.

I admit that I'm very new to the code base and openssl but having a
name like PasswordCallback or something similar would have helped me
so I'm suggesting this change.

PR-URL: https://github.com/nodejs/node/pull/12787


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>