State loading/saving: Instead of a blob, segment into sections marked with tags.

Primary rationale: The part that DHT saves changes if IP is expanded to IPv6. To let people keep their friends/name, change the datafile format now, while everybody is still on the same page.
Loading/Saving rewritten to allow a part of the file to be incomprehensible to the loading routine.
Added a magic cookie at the beginning to mark the file as tox's.
Changes in some part of the datafile can be skipped and the remaining parts still be consumed.
Allows a wide margin to forward compatibility (e.g. if key format is changed).
As long as the file is not completely garbled, the routine will read as much as possible. Only the KEY section is considered mandatory: a malformed key section leads to a negative result.

util.*:
- holds the driving function which jumps from section to section and calls back with section length and tag (type)

Messenger.c,DHT.*:
- new loading functions call the util-function with a callback, which subsequently consumes the sections
- old routines are kept to fall back onto if the magic cookie at the beginning isn't present
- saving is still done in one local routine