MFV r325013,r325034: 640 number_to_scaled_string is duplicated in several commands

illumos/illumos-gate@0a0551200ecbcd4f1b17560acaeeb7b6c8b0090e
https://github.com/illumos/illumos-gate/commit/0a0551200ecbcd4f1b17560acaeeb7b6c8b0090e

https://www.illumos.org/issues/640

du(1), df(1m), ls(1), and swap(1m) all include a copy (it appears literally
copied) of the 'number_to_scaled_string' function in their source. This should
be moved to a shared library and all 4 commands should use this instead.

FreeBSD note: of all libcmdutils functionality ZFS (and other illumos
contrib code) currently uses only nicenum() function (which is similar
to humanize_number but has some formatting differences). For this
reason I decided to not port the whole library. As a result, nicenum.c
from libcmdutils is compiled into libzfs and libzpool. This is a bit
ugly, but works. If one day we are forced to create libillumos, then
the file should be moved to that library.

Reviewed by: Sebastian Wiedenroth <wiedi@frubar.net>
Reviewed by: Robert Mustacchi <rm@joyent.com>
Reviewed by: Yuri Pankov <yuripv@gmx.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: Jason King <jason.brian.king@gmail.com>

MFC after: 2 weeks