hash: allow non-security MD5 use in FIPS mode

gnutls running in the FIPS140-2 mode does not allow MD5 to be
initialized, which breaks chronyd using MD5 to calculate reference ID
of IPv6 addresses. Specify a new hash algorithm for non-security MD5 use
and temporarily switch to the lax mode when initializing the hash
function.

hash_nettle.c

@@ -59,6 +59,9 @@ HSH_GetHashId(HSH_Algorithm algorithm)
 {
   int id, nid;
 
+  if (algorithm == HSH_MD5_NONCRYPTO)
+    algorithm = HSH_MD5;
+
   for (id = 0; hashes[id].algorithm != 0; id++) {
     if (hashes[id].algorithm == algorithm)
       break;