core/net/resolv: IPv6 and mDNS ("Bonjour") support. Major refactor.

This patch updates the DNS resolver to support IPv6 and introduces an
improved API for looking up DNS entries. This patch also adds optional
support for mDNS lookups and responses to the DNS resolver.

Here is a quick summary of the changes:

 * Added support for IPv6 lookups.
 * DNS queries now honor record expiration.
 * Added support for mDNS, compatible with "Bonjour".
 * Implemented a new lookup api, `resolv_lookup2()`, which provides
   more information about the state of the record(error, expired,
   looking-up, etc.).

About mDNS/Bonjour Support
--------------------------

This patch adds basic support for mDNS/Bonjour, which allows you to
refer to the name of a device instead of its IP address. This is
incredibly convenient for IPv6 addresses because they tend to be very
long and difficult to remember. It is especially important for
link-local IPv6 addresses, since not all programs support the '%'
notation for indicating a network interface (required on systems with
more than one network interface to disambiguate).

In other words, instead of typing in this:

 * `http://[fe80::58dc:d7ed:a644:628f%en1]/`

You can type this instead:

 * `http://contiki.local/`

Huge improvement, no?

The convenience extends beyond that: this mechanism can be used for
nodes to talk to each other based on their human-readable names instead
of their IPv6 addresses. So instead of a switch on
`aaaa::58dc:d7ed:a644:628f` triggering an actuator on
`aaaa::ed26:19c1:4bd2:f95b`, `light-switch.local` can trigger the
actuator on `living-room-lights.local`.

What you need to do to be able to look up `.local` names on your
workstation depends on a few factors:

 * Your machine needs to be able to send and receive multicast packets
   to and from the LoWPAN. You can do this easily with the Jackdaw
   firmware on an RZUSBStick. If you have a border router, you will need
   it to bridge the mDNS multicast packets across the border.

 * If you are using a Mac, you win. All Apple devices support mDNS
   lookups.

 * If you are using Windows, you can install Apple's Bonjour for Windows
   package. (This may be already installed on your machine if you have
   installed iTunes) After you install this you can easily do `.local`
   lookups.

 * If you are using a Unix machine, you can install Avahi.

The default hostname is set to `contiki.local.`. You can change the
hostname programmatically by calling `resolv_set_hostname()`. You can
change the default hostname by changing `CONTIKI_CONF_DEFAULT_HOSTNAME`.

You may disable mDNS support by setting `RESOLV_CONF_SUPPORTS_MDNS` to
`0`.