Add support for SIOCGIFXMEDIA to iflib

SIOCGIFXMEDIA is required for extended ethernet media types,
but iflib did not support it.

Reported by: Bhargava Chenna Marreddy <bhargava.marreddy@broadcom.com>
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D13312