Added support for multiple DNS and NBNS servers.

This patch also combines ipv4-dns and ipv6-dns options
that are now handled as aliases to dns.

A side-effect of this patch is that the local keyword is no
longer supported.

doc/sample.config

@@ -137,7 +137,7 @@ socket-file = /var/run/ocserv-socket
 # The user the worker processes will be run as. It should be
 # unique (no other services run as this user).
 run-as-user = nobody
-run-as-group = nobody
+run-as-group = daemon
 
 # Set the protocol-defined priority (SO_PRIORITY) for packets to
 # be sent. That is a number from 0 to 6 with 0 being the lowest
@@ -159,9 +159,10 @@ default-domain = example.com
 
 ipv4-network = 192.168.1.0
 ipv4-netmask = 255.255.255.0
-# Use the keywork local to advertize the local P-t-P address as DNS server
-# ipv4-dns = 192.168.2.1
-ipv4-dns = local
+
+# dns = 192.168.2.1
+dns = 192.168.1.1
+dns = fe80::1
 
 # The NBNS server (if any)
 #ipv4-nbns = 192.168.2.3