Commits on Mar 4, 2016

1. api: add ARES_OPT_NOROTATE optmask value …

   Fix up a couple of problems with configuring whether c-ares rotates
   between different name servers between requests.
   
   Firstly, ares_save_options() returns (in *optmask) the value of
   (channel->optmask & ARES_OPT_ROTATE), which doesn't necessarily
   indicate whether the channel is or is not actually doing rotation.
   This can be confusing/incorrect if:
    - the channel was originally configured without ARES_OPT_ROTATE
      (so it appears that the channel is not rotating)
    - the /etc/resolv.conf file includes the 'rotate' option
      (so the channel is actually performing rotation).
   
   Secondly, it is not possible to reliably configure a channel
   to not-rotate; leaving off ARES_OPT_ROTATE is not enough, since
   a 'rotate' option in /etc/resolv.conf will turn it on again.
   
   Therefore:
    - add an ARES_OPT_NOROTATE optmask value to allow explicit
      configuration of no-rotate behaviour
    - in ares_save_options(), report the value of channel->rotate
      as exactly one of (optmask & ARES_OPT_ROTATE) or
      (optmask & ARES_OPT_NOROTATE).
   
   In terms of back-compatibility:
    - existing apps that set ARES_OPT_ROTATE will continue to rotate,
      and to have ARES_OPT_ROTATE reported back from ares_save_options()
    - existing apps that don't set ARES_OPT_ROTATE will continue to
      use local config/defaults to decide whether to rotate, and will
      now get ARES_OPT_ROTATE or ARES_OPT_NOROTATE reported back from
      ares_save_options() rather than 0.

   

   daviddrysdale committed Mar 4, 2016
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