fix: handle nat66 prefix and unsupported opts in --vmnet-disable-dhcp

Follow-up to Copilot review on the macOS 26 --vmnet-disable-dhcp path:

- Apply --vmnet-nat66-prefix via vmnet_network_configuration_set_ipv6_prefix
  (parsed with inet_pton, /64), mirroring the legacy vmnet_nat66_prefix_key.
- Warn that --vmnet-interface-id and --vmnet-network-identifier are ignored
  on this path: the macOS 26 vmnet network configuration API has no setter
  for the network identifier, and vmnet_interface_start_with_network assigns
  its own interface id (verified empirically on macOS 26.4).
- Validate --vmnet-gateway / --vmnet-mask separately and print the offending
  value instead of relying on errno (inet_aton does not set errno on failure).
- Tighten the disable-dhcp test assertion and PID handling.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: fpolica91

main.c

@@ -271,8 +271,12 @@ static interface_ref start(struct state *state, struct cli_options *cliopt) {
       }
       if (cliopt->vmnet_gateway != NULL) {
         struct in_addr gateway, subnet, mask;
-        if (!inet_aton(cliopt->vmnet_gateway, &gateway) || !inet_aton(cliopt->vmnet_mask, &mask)) {
-          ERRORN("inet_aton");
+        if (!inet_aton(cliopt->vmnet_gateway, &gateway)) {
+          ERRORF("invalid address \"%s\" was specified for --vmnet-gateway", cliopt->vmnet_gateway);
+          return NULL;
+        }
+        if (!inet_aton(cliopt->vmnet_mask, &mask)) {
+          ERRORF("invalid address \"%s\" was specified for --vmnet-mask", cliopt->vmnet_mask);
           return NULL;
         }
         subnet = gateway;

test/test-disable-dhcp.sh

@@ -15,11 +15,15 @@ if [ ! -f ipxe.lkrn ]; then
 	curl -fSL -O https://boot.ipxe.org/ipxe.lkrn
 fi
 
-sudo /opt/socket_vmnet/bin/socket_vmnet --vmnet-disable-dhcp \
+PIDFILE="$(mktemp -t socket_vmnet.pid.XXXXXX)"
+
+sudo /opt/socket_vmnet/bin/socket_vmnet --pidfile "${PIDFILE}" --vmnet-disable-dhcp \
 	--vmnet-gateway="${GATEWAY}" --socket-group=staff "${SOCKET}" &
-socket_vmnet_pid=$!
 cleanup() {
-	sudo kill "${socket_vmnet_pid}" 2>/dev/null || true
+	if [ -f "${PIDFILE}" ]; then
+		sudo kill "$(cat "${PIDFILE}")" 2>/dev/null || true
+		rm -f "${PIDFILE}"
+	fi
 	sudo rm -f "${SOCKET}"
 }
 trap cleanup EXIT