DHCP6: For Prefix Delegation, the - interface means no assignment

- is an invalid interface name.
So we take this to mean don't assign the Delegated Prefix to
any interfaces.
The reject route for the Delegated Prefix is still installed.

Fixes #270

src/dhcp6.c

@@ -3871,8 +3871,9 @@ dhcp6_activateinterfaces(struct interface *ifp)
 			sla = &ia->sla[j];
 			ifd = if_find(ifp->ctx->ifaces, sla->ifname);
 			if (ifd == NULL) {
-				logwarn("%s: cannot delegate to %s",
-				    ifp->name, sla->ifname);
+				if (*sla->ifname != '-')
+					logwarn("%s: cannot delegate to %s",
+					    ifp->name, sla->ifname);
 				continue;
 			}
 			if (!ifd->active) {

src/dhcpcd.conf.5.in

@@ -24,7 +24,7 @@
 .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 .\" SUCH DAMAGE.
 .\"
-.Dd October 4, 2023
+.Dd December 18, 2023
 .Dt DHCPCD.CONF 5
 .Os
 .Sh NAME
@@ -359,6 +359,7 @@ Otherwise addresses are only assigned for each
 .Ar interface
 and
 .Ar sla_id .
+To avoid delegating to any interface, use - as the invalid interface name.
 Each assigned address will have a
 .Ar suffix ,
 defaulting to 1.
@@ -415,6 +416,7 @@ interface eth0
   ia_na 1                # request an IPv6 address
   ia_pd 2 eth1/0         # request a PD and assign it to eth1
   ia_pd 3 eth2/1 eth3/2  # req a PD and assign it to eth2 and eth3
+  ia_pd 4 -              # request a PD but don't assign it
 .Ed
 .It Ic ipv4only
 Only configure IPv4.