Don't bail out without nl80211 (freifunk-gluon#1)

With this change the program just prints an error message when
there is no support for nl80211 in the kernel.

To work properly it is checked wether ctx->wifistations_ctx.fd is
initialized (>= 0) before polling at this file descriptor in
function loop().

src/main.c

@@ -71,7 +71,9 @@ void loop(struct l3ctx *ctx) {
 	add_fd(efd, ctx->icmp6_ctx.nsfd, EPOLLIN);
 	add_fd(efd, ctx->arp_ctx.fd, EPOLLIN);
 	add_fd(efd, ctx->intercom_ctx.fd, EPOLLIN | EPOLLET);
-	add_fd(efd, ctx->wifistations_ctx.fd, EPOLLIN);
+	if (ctx->wifistations_ctx.fd >= 0) {
+		add_fd(efd, ctx->wifistations_ctx.fd, EPOLLIN);
+	}
 
 	/* Buffer where events are returned */
 	events = calloc(maxevents, sizeof(struct epoll_event));

src/wifistations.c

@@ -106,7 +106,13 @@ void wifistations_init(wifistations_ctx *ctx) {
 	int nl80211_id = genl_ctrl_resolve(ctx->nl_sock, "nl80211");
 	if (nl80211_id < 0) {
 		fprintf(stderr, "nl80211 not found.\n");
-		goto fail;
+		/* To resolve issue #29 we do not bail out, but return with an
+		 * invalid file descriptor and without a wifi socket instead.
+		 */
+		ctx->fd = -1;
+		nl_socket_free(ctx->nl_sock);
+		ctx->nl_sock = NULL;
+		return;
 	}
 
 	/* MLME multicast group */