Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Backout libinstall.a -> libpkg commit.

Discussed with: erwin, brooks, bapt

- Take libinstall.a out of pkg_install and make it a proper shared library.
- Rework the wrapper support to check libpkg version as well as pkg_install
version.
- Add libfetch to _prebuild_libs.
- There are no new features introduced.

Notes: the API is not stable, so basically, do not use libpkg in your
projects for now. Also there's no manpage for libpkg yet, because the API
will change drastically. I repeat, do not use libpkg for now.

The last big commit: let usr.sbin/ use WARNS=6 by default.

Some code cleanups, including removing of stale getopt argument and
hardcoded +CONTENTS defines.

PR: bin/117855
Submitted by: Beat Gaetzi <beat@chruetertee.ch>
MFC after: 14 days

Introduce pkg_updating(1) which scans for installed ports and shows
all ports/UPDATING entries that affect one of the installed ports,
and are relevant on the given machine.

PR: bin/117564
Submitted by: Beat Gaetzi <beat@chruetertee.ch>
MFC after: 14 days