dns/blocky: Support running daemon as non-root user

Most rc.d scripts support a standard <service name>_user option in
/etc/rc.conf to run the service as the specified user. The rc.d script
for dns/blocky doesn't observe this setting. As a result, it's not
possible to run as a user other than root (blocky documentation
recommends using a non-privileged user).

Instructions on how to run non-root user daemon have been added to
pkg-message.

PR: 269198
MFH: 2023Q1 (security fixes)