Change PBES2 KDF default salt length to 16 bytes.

Change PBES2 KDF default salt length to 16 bytes.

The PKCS5 (RFC 8018) standard uses a 64 bit salt length for PBE, and
recommends a minimum of 64 bits for PBES2. For FIPS compliance PBKDF2
requires a salt length of 128 bits.
This affects OpenSSL command line applications such as "genrsa" and "pkcs8"
and API's such as PEM_write_bio_PrivateKey() that are reliant on the
default salt length.

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/21858)