Various Coverity fixes in ifconfig(8)

[asomers]

Summary

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)

CID: 1305624, 1305205, 1007100, 1305806, 1305750, 1011474, 270774,
CID: 1011259

[cem]

sbin/ifconfig/af_inet6.c

360

res is uninitialized in some cases.

sbin/ifconfig/ifconfig.c

109

I believe the preferred style for declarations is to put attributes after the declaration. Like:

static void usage(void) _Noreturn;

[asomers]

Respond to cem's comments

[cem]

LGTM other than the whitespace nit below.

sbin/ifconfig/ifconfig.c

109

Well, keep the preexisting space/tab style :-).