DES: support binary keys/plaintext/ciphertext and PKCS#5 padding, improve efficiency and add test script against OpenSSL

[zandbelt]

• parse keys that include 0's e.g. converted from hexadecimal inputs
• handle arbitrary binary plaintext/ciphertext without assuming they should be NULL terminated strings (which are handled properly anyhow)
• support PKCS#5 padding and use memcpy instead of looping over bytes