BUG/MAJOR: dns: restart sockets after fork()

BUG/MAJOR: dns: restart sockets after fork()

UDP sockets used to send DNS queries are created before fork happens and
this is a big problem because all the processes (in case of a
configuration starting multiple processes) share the same socket. Some
processes may consume responses dedicated to an other one, some servers
may be disabled, some IPs changed, etc...

As a workaround, this patch close the existing socket and create a new
one after the fork() has happened.

[wt: backport this to 1.7]