Create a new API to abstract the crypto functions used by pppd.

Create a new API to abstract the crypto functions used by pppd.

This re-introduces the missing DES encryption functions copied from Openssl 3.0 project. Incorporates a new API for performing MD4/MD5/SHA and encryption using DES-ECB mode.

Unit tests are included for respective digest/encryption functions using this new API. With this change, you can pass configure --without-openssl to use the internally provided functions. If you do have openssl, then it will default to use these functions. This also provides a framework to allow other vendors to provide crypto.

This closes #333, partially addresses #242 (except the pkcs11 engine support). Word has it that openssl is working on support for this, and the libp11 / opensc project are inclined not to support this.

Signed-off-by: Eivind Næss <eivnaes@yahoo.com>