Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC: r241737,r246790,r247060,r247841,r248033,r248133,r255468,r256450,r256770
r256971,r256978,r257051,r257142,r257145,r257146,r257147,r257148,r257149
r257158,r257193,r257377,r257378,r257400,r257505,r257668,r257701,r257945
r258020,r258226,r258348,r258550,r259266,r259773,r259774,r259775,r262400
r262401,r262419

Sync pkg(7) with head:
- signature checking
- respecting and reading repository configurations
- support pkg bootstrap -f

Direct modifications:
- Support old libarchive
- Support old openssl API
- Define non yet existing elf macros

MFC: r243883

if PACKAGESITE url scheme is not 'file://', always try to revolv SRV records and
use them if any. It allows the bootstrap to use directly pkg.FreeBSD.org instead
of pkgbeta.FreeBSD.org

MFC r234313 r234315 r234322 r234351 r234870

bootstrap tool for pkgng.
it respects PACKAGESITE, PACKAGEROOT, and a new environment variable ABI (if a
user want to use a different API from the base one for its packages)
it has no man page on purpose to avoid hidding the pkg(8) man page from the
pkgng package.
for now uses pkgbeta.FreeBSD.org as default mirror to find its package
it respects MK_PKGTOOLS

Approved by: des (mentor)

add usr.sbin/pkg which is a bootstrap tool for pkgng.
it respects PACKAGESITE, PACKAGEROOT, and a new environment variable ABI (if a user want to use a different API from the base one for its packages)
it has no man page on purpose to avoid hidding the pkg(8) man page from the pkgng package.
for now uses pkgbeta.FreeBSD.org as default mirror to find its package
it respects MK_PKGTOOLS

Approved by: des (mentor)