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

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

ANSIfy and remove register.

Resulting binary verified with strip(1)+md5(1).

Fix definitions of kilobits etc.

PR: bin/106116
Nudged by: Rostislav Krasny
MFC after: 3 days

Add the Tera scale factor, which is an easy job now.
More scale factors would overflow the command line where
the help for "scale" were shown.

Stop exposing things that can be private to convtbl.c.

Keep all convtbl-related constants and strings in convtbl.[ch].

Allow for large scale factors. C99 warrants that
ULLONG_MAX is not less than 2^64-1; and uintmax_t
cannot be more narrow than unsigned long long.
This allows for scale factors up to Exa inclusively.

Use plain int for the scale index to be consistent
with ifcmds.c and enum.

+ Drop useless initializers.
+ style(9).

Use C99 initializers so that we don't really have to worry
about the order of related things at several places.

There is no reason to use __inline here because we are rather far
from a path critical to performance.

Don't overflow from the gigabyte scale to the bit scale if the
number to auto-scale is >= 1024 Gb. Could be triggered on arches
where ifdata counters had 64 bits.

Reported by: Miroslav Slavkov on -net
MFC after: 3 days

Consistently use __inline instead of __inline__ as the former is an empty macro
in <sys/cdefs.h> for compilers without support for inline.

Fix the easy warnings:

1) Avoid shadowing index.
2) Constness.
3) Missing prototype for ifcmd.
4) Missing include of string.h.
5) Avoid shadowing error function.
6) ANSI definition for main.

Add "ifstat" display:

ifstat Display the network traffic going through active interfaces
on the system. Idle interfaces will not be displayed until
they receive some traffic.

For each interface being displayed, the current, peak and
total statistics are displayed for incoming and outgoing
traffic. By default, the ifstat display will automatically
scale the units being used so that they are in a human-read-
able format. The scaling units used for the current and peak
traffic columns can be altered by the scale command.

Submitted by: Trent Nelson <trent@arpa.com>