MFC r326276:

various: general adoption of SPDX licensing ID tags.

Mainly focus on files that use BSD 2-Clause license, however the tool I
was using misidentified many licenses so this was mostly a manual - error
prone - task.

The Software Package Data Exchange (SPDX) group provides a specification
to make it easier for automated tools to detect and summarize well known
opensource licenses. We are gradually adopting the specification, noting
that the tags are considered only advisory and do not, in any way,
superceed or replace the license texts.

No functional change intended.

Copy head@r302406 to stable/11 as part of the 11.0-RELEASE cycle.
Prune svn:mergeinfo from the new branch, as nothing has been merged
here.

Additional commits post-branch will follow.

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

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.

C can enumerate things for us.

The real contents of this file were repeated twice.
The repetition was harmless due to a usual #ifndef _FOO_H_ wrapper.
Fortunately, nobody started to hack the second copy,
so just remove it from the file.

MFC after: 3 days

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>