core/net/resolv: IPv6 support and improved API. MDNS responder and resolver support.

This patch updates the DNS resolver to support IPv6 and introduces an improved API for looking up DNS entries.

This patch also adds optional suport for MDNS lookups and responses to the DNS resolver.

Here is a quick summary of the changes:

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

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 linux 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`.
examples/webserver: Add MDNS resolver (Bonjour) support.

This allows you to access the web server by simply visiting
<http://contiki.local./>, instead of needing to enter in the
IP address manually.