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.

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>