Handle UDP send errors when sending DNS message larger than MTU

Handle UDP send errors when sending DNS message larger than MTU

When the fragmentation is disabled on UDP sockets, the uv_udp_send()
call can fail with UV_EMSGSIZE for messages larger than path MTU.
Previously, this error would end with just discarding the response.  In
this commit, a proper handling of such case is added and on such error,
a new DNS response with truncated bit set is generated and sent to the
client.

This change allows us to disable the fragmentation on the UDP
sockets again.