Add support for different authentication hashes

Allow different hash functions to be used in the NTP and cmdmon
protocols. This breaks the cmdmon protocol compatibility. Extended key
file format is used to specify the hash functions for chronyd and new
authhash command is added to chronyc. MD5 is the default and the only
function included in the chrony source code, other functions will be
available from libraries.

hash.h

@@ -0,0 +1,41 @@
+/*
+  chronyd/chronyc - Programs for keeping computer clocks accurate.
+
+ **********************************************************************
+ * Copyright (C) Miroslav Lichvar  2011
+ * 
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of version 2 of the GNU General Public License as
+ * published by the Free Software Foundation.
+ * 
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, USA.
+ * 
+ **********************************************************************
+
+  =======================================================================
+
+  Header file for crypto hashing.
+
+  */
+
+#ifndef GOT_HASH_H
+#define GOT_HASH_H
+
+/* length of hash values produced by SHA512 */
+#define MAX_HASH_LENGTH 64
+
+extern int HSH_GetHashId(const char *name);
+
+extern unsigned int HSH_Hash(int id,
+    const unsigned char *in1, unsigned int in1_len,
+    const unsigned char *in2, unsigned int in2_len,
+    unsigned char *out, unsigned int out_len);
+
+#endif