make scrypt ASN.1 parameter functions public

Since scrypt PBKDF can be used both in PKCS#5 and PKCS#12 files,
do share the code between them.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1334)