MFC r317755, r317758

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

git-svn-id: svn://svn.freebsd.org/base/stable/10@319265 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f