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 r289362, r289445: 2605 want to resume interrupted zfs send

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed by: Xin Li <delphij@freebsd.org>
Reviewed by: Arne Jansen <sensille@gmx.net>
Approved by: Dan McDonald <danmcd@omniti.com>
Author: Matthew Ahrens <mahrens@delphix.com>

illumos/illumos-gate@9c3fd1216fa7fb02cfbc78a2518a686d54b48ab8

For more info, see:
- slides http://www.slideshare.net/MatthewAhrens/openzfs-send-and-receive
- video https://www.youtube.com/watch?v=iY44jPMvxog
- manpage changes (for zfs resume -s and zfs send -t)
- upcoming talk at the OpenZFS Developer Summit

The TL;DR is:
Use "zfs receive -s" to save the partially received state on failure.
On failure, get the receive token with "zfs get receive_resume_token <fs>"
Resume the send with "zfs send -t <token_value>"

Relnotes: yes

MFC r284409: Fix circular dependency between libzfs and libzfs_core

By bapt.

MFC r272484:

Add dependencies to various libraries to libzfs and libzpool.

Submitted by: sef

MFC r266520:

Explicitly link libzfs against libavl as it is done in OpenSolaris
(4543:12bb2876a62e). Without this, some third party applications
may break because the lack of AVL related symbols.

FreeBSD base system are not affected because the FreeBSD ZFS command
line tools were all linked against libavl and thus hide the underlying
issue.

PR: bin/183081

MFC r265689:
Import adapted OpenSolaris' thread pool API implementation.

The thread pool is used by libzfs to implement parallel disk scanning.
Without this change our dummy wrapper made `zpool import ZZZ` command to
scan all disks sequentially from the single thread when searching for pools.
This change makes it use two threads per CPU, same as in OpenSolaris.

On system with 200 HDDs this change reduces ZFS pool import time from 35
to 22 seconds.

MFC r272484:

Add dependencies to various libraries to libzfs and libzpool.

Submitted by: sef

MFC r266520:

Explicitly link libzfs against libavl as it is done in OpenSolaris
(4543:12bb2876a62e). Without this, some third party applications
may break because the lack of AVL related symbols.

FreeBSD base system are not affected because the FreeBSD ZFS command
line tools were all linked against libavl and thus hide the underlying
issue.

PR: bin/183081

MFC r265689:
Import adapted OpenSolaris' thread pool API implementation.

The thread pool is used by libzfs to implement parallel disk scanning.
Without this change our dummy wrapper made `zpool import ZZZ` command to
scan all disks sequentially from the single thread when searching for pools.
This change makes it use two threads per CPU, same as in OpenSolaris.

On system with 200 HDDs this change reduces ZFS pool import time from 35
to 22 seconds.