MFC: r256770,r257142,r257145,r257146,r257147,r257148,
     r257149,r257150,r257158,r257159,r257164,r257168,
     r257193

  - Support checking signature for pkg bootstrap from remote
    and for 'pkg add ./pkg.txz'

  - Be verbose on where pkg is being bootstrapped from.

  - Add support for reading configuration files from /etc/pkg.
    For now only /etc/pkg/FreeBSD.conf is supported.

  - Add test package signing key fingerprint into /etc/keys/pkg/trusted.

  - Disable fingerprint checking by default for now as the official
    packages are not yet signed.

Approved by:	bapt
Approved by:	re (glebius)
MFC r257151:

  Document /var/cache/pkg into hier(7) which pkg(8) uses.

Approved by:	bapt
Approved by:	re (glebius)