pkcs5v2: Add pkcs5v2_genkey_raw function

This is like pkcs5v2_genkey but takes a "passphrase" as a buffer and
length rather than a NUL-terminated string.

Reviwed by: pjd
MFC after: 1 week
Sponsored by: Amazon
Differential Revision: https://reviews.freebsd.org/D46633

(cherry picked from commit 8ce291a335bc751e7a89691eebfb9f9da65b82ac)