Finish crypto wrapping. Also provide wrappers for OpenSSL.

Disable libgcrypt by default. Since it doesn't support the OFB cipher mode,
we can't use it in a backwards compatible way.

src/node.h

@@ -24,7 +24,9 @@
 #define __TINC_NODE_H__
 
 #include "splay_tree.h"
+#include "cipher.h"
 #include "connection.h"
+#include "digest.h"
 #include "list.h"
 #include "subnet.h"
 
@@ -50,13 +52,9 @@ typedef struct node_t {
 
 	node_status_t status;
 
-	const EVP_CIPHER *cipher;		/* Cipher type for UDP packets */
-	char *key;				/* Cipher key and iv */
-	int keylength;				/* Cipher key and iv length */
-	EVP_CIPHER_CTX packet_ctx;		/* Cipher context */
-	
-	const EVP_MD *digest;			/* Digest type for MAC */
-	int maclength;				/* Length of MAC */
+	cipher_t cipher;                        /* Cipher for UDP packets */
+	digest_t digest;                        /* Digest for UDP packets */	
+	int maclength;				/* Portion of digest to use */
 
 	int compression;			/* Compressionlevel, 0 = no compression */