resolved: unify the two functions to create extra stubs

resolved: unify the two functions to create extra stubs

There is a minor functional change:
IPV6_FREEBIND is set of IPv6 sockets, not IP_FREEBIND. This was missed in
af8b1384, but I noticed only after the merging the two functions.

And a not-so-minor functional chagnge:
7216a3b5dcde36245 changed manager_dns_stub_tcp_fd_extra() to return the fd even
if the source was already initialized, but it didn't do the same change for
manager_dns_stub_udp_fd_extra(), so it would return 0 in that case. But
0354029bf572489b uses manager_dns_stub_udp_fd_extra() when preparing to call
manager_send(), and will pass 0 as the fd in that case. For both socket types
fd is now always returned.