Various Coverity fixes in ifconfig(8)

• Exit early if kldload(2) fails (1011259). This is the only change that affects ifconfig's behavior.
• Close memory and resource leaks (1305624, 1305205, 1007100)
• Mark usage() as _Noreturn (1305806, 1305750)
• Fix some dereference after null checks (1011474, 270774)

Reported by: Coverity
CID: 1305624, 1305205, 1007100, 1305806, 1305750, 1011474,
CID: 270774, 1011259
Reviewed by: cem
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D10587