Insta-MFC r269489: partial revert of r262867 which was MFC'ed as r263820.
Don't ignore sndbuf/rcvbuf limits for SOCK_DGRAM sockets.  This appears
to be an edit error or patch fuzz mismatch.
MFC	r268945:
Fix hdestroy() compliance issue.

The hcreate(3) implementation and related functions we inherited
from NetBSD used to free() the key value, something that is not
supported by the standard implementation.

This would cause a segmentation fault when attempting to run
the examples from the opengroup and linux manpages.

There is no need to bump the __FreeBSD_version as we have
always claimed XPG4.2 compliance but if some reference is
required, the bump for r269484 can be used.

Reference:
http://bugs.dragonflybsd.org/issues/1398
MFC r269187:
Assert that nullfs vnode has VV_ROOT set whenever lower vnode has.
Assert that dotdot lookup on the root vnode is not performed.
MFC r269189:
Initialize zfs vnode v_hash when the vnode is allocated.
MFC r269190:
For md(4), posix shm(3) and tmpfs(5), free swap space used by paged in
dirty page, which is written by the process.
MFC r269289, r269290, r269291, r269292, r269293:

r269289:
  sort(1): Remove trailing '.' from See Also section.

r269290:
  acpi_wmi(4): Remove trailing comma from standalone Xref.

r269291:
  hptiop(4): Remove trailing comma from ending Xref.

r269292:
  pf.conf(5): Remove trailing comma from ending Xref.

r269293:
  kernel_mount(9): Remove trailing comma from ending Xref.

Sponsored by:	The FreeBSD Foundation