MFC r257037: Redefine the io provider using the SDT(9) macros

MFC slacker:	markj
MFC r258605: Convert over the TCP probes to use mtod()

MFC slacker:	adrian
MFC r258622: dtrace sdt: remove the ugly sname parameter of SDT_PROBE_DEFINE
MFC of 260079:

Properly handle unsigned comparison.
MFC of 260088:

Fine tune filesystem block allocations under low free-space
conditions (-r254995) based on further operational experience.

Submitted by:  Dmitry Sivachenko
Fix Tested by: Dmitry Sivachenko
Document the killall(1) regression.

While I'm there also correct a typo.

Reviewed by:	gjb (earlier version), hrs
MFC r260566: Remove incorrect bit shift when assigning the LUN request field
Document possible poor NFSv6 performance with bxe(4) when 'tso'
is enabled.

Approved by:	re (implicit)
Sponsored by:	The FreeBSD Foundation
Document a bsdconfig(8) compatibility nit with pkg(7) 1.2.x.

Approved by:	re (implicit)
Sponsored by:	The FreeBSD Foundation
Fix wording from r260842.

Approved by:	re (implicit)
Sponsored by:	The FreeBSD Foundation
Document pw(8) regression when the '-G' flag is used.

Approved by:	re (implicit)
Sponsored by:	The FreeBSD Foundation
Fix description of pw(8) regression.

Approved by:	re (implicit)
Sponsored by:	The FreeBSD Foundation
MFC r260581: Add sglist_append_bio(9) to append a struct bio's data to
a sglist
MFC r260582: Add unmapped IO support to virtio_blk(4)
MFC r260583: Add unmapped IO support to virtio_scsi(4)
MFC r258675: Fix build.
MFC r260556: Add some missing .Nm for newer syscalls in existing man pages.
- Fix a bxe(4) entry.  This issue is not related to NFSv4, and poor
  performance is caused by disabling TSO, not the issue itself.
- s/&os;-STABLE/&os; &release.current;/
Document an ipfw fwd issue.

Suggested by:	jmg