config: set RFC defaults for preferred lifetime

Update preferred lifetime default value in accordance with RFC4861.

Signed-off-by: Paul Donald <newtwen@gmail.com>
[ fix comment style ]
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>

README

@@ -116,7 +116,7 @@ domain			list	<local search domain>	Search domains to announce
 leasetime		string	12h			DHCPv4 address leasetime
 start			integer	100			DHCPv4 pool start
 limit			integer	150			DHCPv4 pool size
-preferred_lifetime	string	12h			Value for the preferred lifetime
+preferred_lifetime	string	7d			Value for the preferred lifetime
 							for a prefix
 ra_default		integer	0			Override default route
 			0: default, 1: ignore no public address, 2: ignore all

src/config.c

@@ -215,7 +215,7 @@ static void set_interface_defaults(struct interface *iface)
 	iface->ndp = MODE_DISABLED;
 	iface->learn_routes = 1;
 	iface->dhcp_leasetime = 43200;
-	iface->preferred_lifetime = 43200;
+	iface->preferred_lifetime = 604800; /* rfc4861#section-6.2.1: AdvPreferredLifetime 7 days */
 	iface->dhcpv4_start.s_addr = htonl(START_DEFAULT);
 	iface->dhcpv4_end.s_addr = htonl(START_DEFAULT + LIMIT_DEFAULT - 1);
 	iface->dhcpv6_assignall = true;