MFC r317755, r317758

r317755:
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
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D10587

r317758:
Unbreak ifconfig for mlx4en(4) after r317755

ifconfig doesn't correctly infer mlx interfaces' module names, so it will
attempt to load the mlx(4) module even when not necessary.

Reported by: rstone
X-MFC-With: 317755
Sponsored by: Spectra Logic Corp