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

MFC r288305:

Replace N #defines with nitems to simplify ifconfig code slightly

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Make it possible to switch pfsync(4) deferral mechanism on/off.

Obtained from: OpenBSD

Print pfsync interface status if either syncpeer or syncdev is configured.

Update ifconfig's pfsync module for 3.7 pfsync(4)

Use afswch->af_other_status for carp_status() and pfsync_status().

Sponsored by: Rambler

Add CARP (Common Address Redundancy Protocol), which allows multiple
hosts to share an IP address, providing high availability and load
balancing.

Original work on CARP done by Michael Shalayeff, with many
additions by Marco Pfatschbacher and Ryan McBride.

FreeBSD port done solely by Max Laier.

Patch by: mlaier
Obtained from: OpenBSD (mickey, mcbride)