Add tox_wait() for socket, allow tox.h include in tox.c

tox.*, Messenger.*, network.*;
- new function tox_wait() => waitMessenger() => networking_wait():
  - waits for a given amount of time for any sort of socket activity
  - mustn't change anything inside, mustn't poll or do other stuff
    - the point is to be able to call this asynchronously from a second thread
    - the outside shall follow up with an immediate tox_do() when there's socket activity

tox.*:
- allow inclusion of tox.h into tox.c, ensuring that the outside interface and the internal always match

util.c:
- squished a few warnings about questionable format strings