usr.sbin/ppp: correct SIOCDIFADDR{,_IN6} calls to use {,in6_}ifreq

The SIOCDIFADDR{,_IN6} ioctls take an ifreq structure object, not an
ifaliasreq structure object, as their argument.

Reviewed by: brooks, jhb, oshogbo
Approved by: oshogbo (mentor)
Obtained from: CheriBSD
Differential Revision: https://reviews.freebsd.org/D46018