Add $DHCPDNS for forwarding rules as auto detected DNS from routers

[lifenjoiner]

New feature born from https://github.com/DNSCrypt/dnscrypt-proxy/discussions/2368, hoping to gain more attentions to be tested.

Especially in scenarios:

1. IPv6 DHCP.
2. Running as a non-root user on Linux.

Pre-built binaries can be downloaded from my branch, or build the dhcpdns branch yourself.