dnscrypt-proxy service: update AppArmor profile

This patch fixes the AppArmor profile path clause and adds
(currently ignored) network rules.

The AppArmor profile used to be defined for the path sbin/dnscrypt-proxy,
but the real path is bin/dnscrypt-proxy (due to sbin now being a symlink
to bin), which permitted the service to run unconfined.

Adding the network rules has no effect other than improving correctness,
as the version of AppArmor in the NixOS kernel fails to enforce network
rules.