core/proxyvm: allow TCP traffic to DNS servers
core/proxyvm: allow TCP traffic to DNS servers

Some DNS queries requires TCP - namely those with response not fitting
in 512 bytes.

woju/qubes-core-admin,woju/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,woju/qubes-core-admin,QubesOS/qubes-core-admin,marmarek/qubes-core-admin,QubesOS/qubes-core-admin,woju/qubes-core-admin