Remove obtained, but never used data.

Found by:	gcc
Remove unsigned comparison < 0

Found by:	LLVM
Reviewed by:	zbb
  Rewrite netstat/if.c to use getifaddrs(3) and getifmaddrs(3) instead of
libkvm digging in kernel memory. This is possible since r231506 made
getifaddrs(3) to supply if_data for each ifaddr.

  The pros of this change is that now netstat(1) doesn't know about kernel
struct ifnet and struct ifaddr. And these structs are about to change
significantly in head soon. New netstat binary will work well with 10.0
and any future kernel.

  The cons is that now it isn't possible to obtain interface statistics
from a vmcore.

  Functions intpr() and sidewaysintpr() were rewritten from scratch.

  The output of netstat(1) has underwent the following changes:

1) The MTU is not printed for protocol addresses, since it has no notion.
   Dash is printed instead. If there would be a strong desire to return
   previous output, it is doable.
2) Output interface queue drops are not printed. Currently this data isn't
   available to userland via any API. We plan to drop 'struct ifqueue' from
   'struct ifnet' very soon, so old kvm(3) access to queue drops is soon
   to be broken, too. The plan is that drivers would handle their queues
   theirselves and a new field in if_data would be updated in case of drops.
3) In-kernel reference count for multicast addresses isn't printed. I doubt
   that anyone used it. Anyway, netstat(1) is sysadmin tool, not kernel
   debugger.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
- While we are spreading the counter(9) across network stack, more userland
tools would need to know about the counter_u64_t type. Allow to include
sys/counter.h from userspace.
- Utilize now defined type in kvm_counter_u64_fetch().

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Now counter_u64_t is known to userland, thus remove hack from r253086.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Remove useless check of ia6 against NULL, right after dereferencing it.
Remove unsigned < 0 check.