1) [DONE] split functions from kex*[cs].c into callbacks similar to kexgexc.c
   and remove packet_read_expect.

2) [DONE] move to session state
	struct session_state {
		Kex			kex;
		Newkeys			*current_keys; /* cf kex.c */
		dispatch_fn		dispatch[]
		int			datafellows
	}

3) [DONE] Code for parsing the keys

4) [DONE] Implement callback for kex->verify_host_key for checking key

5) [DONE] Implement callbacks for:
	kex->load_host_public_key
	kex->load_host_private_key

5) [DONE] figure out how to integrate the banner exchange into ssh_packet_get()

6) [DONE] figure out how integrate kex into ssh_packet_get()

7) [DONE] figure out initial API

8) figure out error-handling and remove fatal()

9) [DONE] Rework privsep's interaction with packet.c: factor everything into
   packet_get_state() and packet_set_state() functions