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

Repo: {
   URL: "...",
   MIRROR_TYPE: "...",
   ...
}

The configuration will be read from /usr/local/etc/pkg.conf if exists,
otherwise /etc/pkg/FreeBSD.conf

Approved by:	bapt
MFC after: 	2 days
Be verbose and tell where pkg(8) is being bootstrapped from.

Approved by:	bapt
MFC after:	2 days
Support checking signature for pkg bootstrap.

If the pkg.conf is configured with SIGNATURE_TYPE: FINGERPRINTS,
and FINGERPRINTS: /etc/keys/pkg then a pkg.sig file is fetched along
with pkg.txz. The signature contains the signature provided by the
signing server, and the public key. The .sig is the exact output
from the signing server in the following format:

  SIGNATURE
  <openssl signed>
  CERT
  <rsa public key>
  END

The signature is verified with the following logic:

 - If the .sig file is missing, it fails.
 - If the .sig doesn't validate, it fails.
 - If the public key in the .sig is not in the known trusted fingerprints,
   it fails.
 - If the public key is in the revoked key list, it fails.

Approved by:	bapt
MFC after:	2 days
Discussed by:	bapt with des, jonathan, gavin
Tell which fingerprint pkg is being validated against.

Approved by:	bapt
MFC after:	2 days
Add support to check the signature of a local pkg.txz file being
added with "pkg add". If the pkg.conf is configured to check for
signature, then the pkg.txz.sig file will be expected and validated
per r257147

Approved by:	bapt
MFC after:	2 days
Add infrastructure for installing pkg(8) keys into /etc/keys/pkg and add
the current test key that packages will be signed with until 10.0-RELEASE.

Approved by:	bapt
Discussed by:	bapt with des
MFC after:	2 days
Document /var/cache/pkg into hier(7) which pkg(8) uses.

Approved by:	bapt
MFC after:	2 days