resolved: choose correct file descriptor for proxy stub replies

resolved: choose correct file descriptor for proxy stub replies

find_socket_fd() does not expect the sender address, but the
listen-address. This is in fact the destination of the DNS packet.
Matching via sender address caused a fallback to the default stub
listener in manager_dns_stub_fd() as the sender address can never
match the proxy stub listen address.

Note that manager_dns_stub_fd() is only used for the default
listener stub and the proxy stub, that means *extra* listeners
stubs (DNSStubListenerExtra=…) have not been affected as
`struct DnsStubListenerExtra` provides a direct link to the event
source.

By using the correct fd we ensure the correct socket options
(like TTL) are used and prevent issues like #23495 in case ifindex
could not be determined.