sbin/nos-tun: correct SIOCDIFADDR calls to use ifreq

The SIOCDIFADDR ioctl takes an ifreq structure object, not an ifaliasreq
structure object, as its argument.

Reviewed by: brooks, jhb, oshogbo
Approved by: oshogbo (mentor)
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D46017