Make ifconfig always exit with an error code if an important ioctl fails

PR: 203062
Arm Twisting by: Kristof Provost
Reviewed by: kp
Approved by: bapt (mentor)
MFC after: 2 weeks
Relnotes: yes
Sponsored by: ScaleEngine Inc.
Sponsored by: vBSDCon
Differential Revision: https://reviews.freebsd.org/D3644