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

Move /etc/keys to /usr/share/keys where users are less likely to modify them.

Requested by: secteam (cperciva, des)
Approved by: bapt

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