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.

MFC r314654:

cddl: normalize paths using SRCTOP-relative paths or :H when possible

This simplifies make logic/output

While here, remove bogus CFLAGS which look for headers in cddl/lib/libumem.
There aren't any source files there (just Makefiles)

MFC r268075: MFV r267565:

4757 ZFS embedded-data block pointers ("zero block compression")
4913 zfs release should not be subject to space checks

MFC r268075: MFV r267565:

4757 ZFS embedded-data block pointers ("zero block compression")
4913 zfs release should not be subject to space checks