MFC r325035: MFV r325013,r325034: 640 number_to_scaled_string is duplicated in several commands

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.