kbd_attach() is called kbd[0-9]+, with a different unit number. This
makes it impossible to write a devd rule which will automatically
switch to a USB keyboard when one is attached, because there is no way
to guess the correct device node to pass to kbdcontrol.
Therefore, change kbd_attach() to create a device node using the
keyboard device's real name (atkbd0, ukbd0...), and create the
kbd[0-9]+ node as an alias for backward compatibility.