Various changes to nTox including a potential crash (also possible from remote!)...

... and a tiny change to toxcore.

Crash stuff:

nTox.c:
- do_refresh(): avoid crashes (input a "%s" and the client goes "boom!", send someone a string with embedded "%s" and see him blow up...)

Other stuff:

toxcore: tox.h (doc.)/network.c (code):
- networking_wait_prepare(): return -1 if lenptr was NULL and therefore not settable

nTox.c:
- fraddr_to_str(): function to convert a TOX_FRIEND_ADDRESS into a segmented (and therefore line-breakable) string
- print_friendlist(): print index of friend on name line, print id on 2nd line
- command /f: skip spaces (and +) inside a friend id
- command /r (new): "/r #" to remove a friend
- main(): reduce cpu consumption if we're not currently sending files