move set_nonblock, create_socket and path_exists to libi3

libi3/nonblock.c

@@ -0,0 +1,20 @@
+#include "libi3.h"
+
+#include <err.h>
+#include <fcntl.h>
+
+/*
+ * Puts the given socket file descriptor into non-blocking mode or dies if
+ * setting O_NONBLOCK failed. Non-blocking sockets are a good idea for our
+ * IPC model because we should by no means block the window manager.
+ *
+ */
+void set_nonblock(int sockfd) {
+    int flags = fcntl(sockfd, F_GETFL, 0);
+    if (flags & O_NONBLOCK) {
+        return;
+    }
+    flags |= O_NONBLOCK;
+    if (fcntl(sockfd, F_SETFL, flags) < 0)
+        err(-1, "Could not set O_NONBLOCK");
+}