MFC	r251809, r251952, r252012:

Re-sort ext2fs headers to make things easier to find.

In the ext2fs driver we have a mixture of headers:

- The ext2_ prefixed headers have strong influence from NetBSD
and are carry specific ext2/3/4 information.
- The unprefixed headers are inspired on UFS and carry implementation
specific information.

Do some small adjustments so that the information is easier to
find coming from either UFS or the NetBSD implementation.

Also
- Set MAXMNTLEN nearer to where it is used.
- Move EXT2_LINK_MAX to ext2_dir.h .
- Rename some prefixes in the Block Group Descriptor fields.
MFC r251799:

Return ENETDOWN when the parent interface is down.
MFC: r251585
Modify nfsstat.c so that it prints out an error when a non-root
user attempts to use the "-m" option and it fails.
MFC r251820.  This fixes some symbol visibility issues with libsupc++ that broke
 compilation of GNUstep libobjc.
MFC r252042:
	Ports switched from dialog to dialog4ports some time ago.

PR:		docs/179785
MFC r251872:
Be sure to actually decrement the "count" parameter for each processed
descriptor so that we return when the threshold has been reached.
MFC r251478: MFV r251474:

 * Illumos zfs issue #3137 L2ARC compression

Whether or not to compress buffers entering the L2ARC is
controlled by "compression" setting on the dataset, when
compression is not "off", L2ARC compression is enabled.

The compress method is always LZ4 for L2ARC when enabled
because it works best for the scenario.
MFC r251520: MFV r251519:

 * Illumos ZFS issue #3805 arc shouldn't cache freed blocks
Merge r243151:
  o Remove meaningless PROTOCOLS section.
  o Describe passing file descriptors as separate section.
    - Descriptors can be passed through any protocol of the UNIX family,
      not SOCK_STREAM and SOCK_SEQPACKET only. [1]
  o Describe socket options as separate section.
    - Move LOCAL_PEERCRED option to this section.
    - Describe struct xucred.
    - In LOCAL_CREDS section mention that credentials
      are passed only on the first read on non-datagram
      sockets.
  o Xref all mentioned system calls.

  Noticed by:   Igor Sysoev [1]
Merge r243569, r243601:
  Re-initialize eh pointer after m_adj(), after ng_ether(4) and
  if_bridge(4) processing.

  Submitted by: Kohji Okuno <okuno.kohji jp.panasonic.com>
  Reviewed by:  yongari
Merge r247910:
  Plug a memory leak.

  Reviewed by:  mav
  Sponsored by: Nginx, Inc.
Merge r247911,r248187,r248254,r250797:

Various quirks to enable headphones redirection on
various Lenovo laptops: X220, X1, X1 Carbon, T420,
T430, T430S, T520.
Merge r248155:
  Reinitialize eh after pfil(9) processing.

  PR:           176764
  Submitted by: adri
Merge r248158:
  Remove LIBALIAS_LOCK_ASSERT(), including a couple with an uninitialzed
  argument, in code that isn't compiled in kernel.

  PR:           kern/176667
  Sponsored by: Nginx, Inc.
Merge r248159:
  Fix for quite a special case when userland emulates a netgraph node, and
  userland can reply to a message with NGM_HASREPLY bit set. In this case
  we should not wait for a response to a responce.

  PR:           176771
  Submitted by: Keith Reynolds <keith.reynolds tidalscale.com>
Merge r248160:
  Fix spelling.

  PR:           176777
  Submitted by: Andrey Simonenko <simon comsys.ntu-kpi.kiev.ua>
Merge r248370:
  Belatedly remove the vinum(4) manual page. The vinum
  manager is absent in FreeBSD since 6.0-RELEASE.

  Reviewed by:  joel
Merge r248489:
  There are actually two different cases when mlock(2) returns
  ENOMEM. Clarify this, taking text from SUS.

  Reviewed by:  kib
Merge r248507:
  iwn(4) doesn't support adhoc mode.

  PR:           misc/177106
  Submitted by: Hiren Panchasara <hiren.panchasara gmail.com>
Merge r248570, 248582:
  Add NGM_NAT_LIBALIAS_INFO command, that reports internal stats
  of libalias instance. To be used in the mpd5 daemon.

  Submitted by: Dmitry Luhtionov <dmitryluhtionov gmail.com>