Added 'saltlen' option to the OpenSSL enc command line app.

Added 'saltlen' option to the OpenSSL enc command line app.

This allows PBKDF2 to change the saltlen to something other than the
new default value of 16. Previously this app hardwired the salt length
to a maximum of 8 bytes. Non PBKDF2 mode uses EVP_BytesToKey()
internally, which is documented to only allow 8 bytes.

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