Simplify sendmmsg code

[Florian Weimer]

On 05/09/2017 08:41 PM, Adhemerval Zanella wrote:
> I think we can add a sysdeps/posix sendmmsg based on sendmsg so we can
> safely remove this macro usage on resolv code.  I will work on this
> after your patch inclusion.

The use of sendmmsg in the resolv code is problematic.  We need to use 
two separate sockets with different source ports to improve 
compatibility anyway.  This might also help firewalls with tearing down 
state more quickly (single request/response very likely does not bump 
the UDP timeout, but two requests and two responses might switch into 
stream mode with a prolonged timeout; maybe that's why we originally hit 
the Netfilter table overflow in some of the glibc resolver tests).

I'm not sure how quickly I will get to fixing this (I hope for 2.26), so 
please do whatever you feel is necessary in the meantime.

Thanks,
Florian