OpenZFS 640 - number_to_scaled_string is duplicated in several commands

OpenZFS 640 - number_to_scaled_string is duplicated in several commands

Porting Notes:
- The OpenZFS patch added nicenum_scale() and nicenum() to a
  library not used by ZFS.  Rather than pull in a new dependency
  the version of nicenum in lib/libzpool/util.c was simply
  replaced with the new one.

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>
Authored by: Jason King <jason.brian.king@gmail.com>
Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
OpenZFS-issue: https://www.illumos.org/issues/640
OpenZFS-commit: https://github.com/openzfs/openzfs/commit/0a055120
Closes #6796