MFC Pkg configuration, known public key, and pkg(7) changes to align with pkg(8)

MFC Pkg configuration, known public key, and pkg(7) changes to align with pkg(8)

This partially merges:
    r229068,r237795,r252048,r257145,r257147,r257150,r257150,r257159,r257164,
    r257168,r257344,r257344,r257667,r257668,r258227,r258550,r263937,r264420

- etc/
  - Bring in current pkg configuration from head
  - Add /etc/pkg/ and /usr/share/keys to mtree
- share/keys
  - Bring in trusted key fingerprint from head
- share/man/man7
  - Document /usr/share/keys/pkg
- usr.sbin/pkg
  - No longer create pkg.conf as it is deprecated in pkg(8).
  - Show security warning when bootstrapping.
    * This is a direct commit as the signature verification is not being
      MFC'd due to being too large.

Discussed with:	bapt, gjb

git-svn-id: svn://svn.freebsd.org/base/stable/8@264519 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f