Store sessions in an RB tree by name rather than a list, this is tidier

and allows them to easily be shown sorted in various lists
(list-sessions/choose-sessions).

Keep a session index which is used in a couple of places internally but
make it an ever-increasing number rather than filling in gaps with new
sessions.

cmd-attach-session.c

@@ -47,7 +47,7 @@ cmd_attach_session_exec(struct cmd *self, struct cmd_ctx *ctx)
 	char			*overrides, *cause;
 	u_int			 i;
 
-	if (ARRAY_LENGTH(&sessions) == 0) {
+	if (RB_EMPTY(&sessions)) {
 		ctx->error(ctx, "no sessions");
 		return (-1);
 	}