Fixed spam problem. (I broke the API so this will not build)

The friend address is what the byte string that you give away for people
to add you will be called.

1. Every friend address now contains a number set by the friend. This is
to prevent someone from randomly spamming people in the DHT with friend
requests and makes it so you need the person to actually give you the
address in some way to send the friend request. This number is expected
to be encrypted with the friend request. All requests that do not
contain this number will be rejected. This means the spammer can no
longer use the DHT to collect lists of valid addresses to spam. It also
enables users to quickly change the number in case a spammer gets hold
of the address and starts spamming it.

2. A 2 byte checksum will be added (not implemented yet) to prevent
people from accidentally adding random strings as friends. (NOTE that
this has nothing to do with the spam problem I just decided to add a
placeholder for it now.)