/etc/netif.d/tap still uses the old tunctl command functionality, which got replaced by the /sbin/ip command from sys-apps/iproute2 package.
The new command is being used like this:
$ ip tuntap add dev $interface mode tap
$ ip link set $interface address $mac_addr
.... and so on

To use the /sbin/ip command functionality, the new /etc/netif.d/tap file should look like this:
#!/bin/sh

netif_create()

{ if [ -n "$user" ]; then ip tuntap add dev $interface mode tap user $user; elif [ -n "$group" ]; then ip tuntap add dev $interface mode tap group $group; else ip tuntap add dev $interface mode tap; fi }

netif_destroy()

{ ip link del $interface; }

netif_pre_up()

{ if [ -n "$mac_addr" ]; then ip link set $interface address $mac_addr; fi ip link set $interface promisc on; }