edns-addr allow CIDR notation, do not assume /24

`dnsproxy -v -l 127.0.0.1 -u tls://dns11.quad9.net --edns --edns-addr=1.2.3.4/16`

Results in error:

`2021/10/12 13:06:55 49029#1 [fatal] cannot parse 1.2.3.4/16`

dnsproxy should not always assume /24 (see `dnsmasq --add-subnet`)