vpn backend: tun interface addresses are incorrect on Darwin

[scareything]

The inet and inet6 address assignments on Darwin are incorrect.

1. The mask on the inet address is incorrect. for example, the command that vnet runs:

    ifconfig utun5 inet 10.122.0.1 10.122.0.1 up
   

   doesn't specify a mask at all, so you get the default mask for the address class - in this case 255.0.0.0 - regardless of the intended subnet size

    utun5: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1500
        inet 10.122.0.1 --> 10.122.0.1 netmask 0xff000000
   

   The mask can be specified with the source address in the ifconfig command:

    ifconfig utun5 inet 10.122.0.1/16 10.122.0.1 up
   
    utun5: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> mtu 1500
            inet 10.122.0.1 --> 10.122.0.1 netmask 0xffff000000
   

2. The inet6 address is not being added at all. It's easy to miss because it's logged as an INFO message, but zrok logs an error:

    2024/04/17 13:55:14.304	INFO	exec ifconfig [utun5 inet6 fd00:7a72:6f6b::1 fd00:7a72:6f6b::1 up]
    2024/04/17 13:55:14.315	INFO	failed to exec cmd: exit status 1