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: r257145,r257147,r257150,r257159,r257164,r257168,r257344,r257667,r257668,
r258227,r260605

Add default configuration for FreeBSD official pkg(8) repositories
Add fingerprint for the trusted keys

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