Move ipc_send_message and ipc_recv_message to libi3

Make i3-msg and src/ipc.c use it

include/libi3.h

@@ -42,4 +42,29 @@ void *srealloc(void *ptr, size_t size);
  */
 char *sstrdup(const char *str);
 
+/**
+ * Formats a message (payload) of the given size and type and sends it to i3 via
+ * the given socket file descriptor.
+ *
+ * Returns -1 when write() fails, errno will remain.
+ * Returns 0 on success.
+ *
+ */
+int ipc_send_message(int sockfd, uint32_t message_size,
+                     uint32_t message_type, const uint8_t *payload);
+
+/**
+ * Reads a message from the given socket file descriptor and stores its length
+ * (reply_length) as well as a pointer to its contents (reply).
+ *
+ * Returns -1 when read() fails, errno will remain.
+ * Returns -2 when the IPC protocol is violated (invalid magic, unexpected
+ * message type, EOF instead of a message). Additionally, the error will be
+ * printed to stderr.
+ * Returns 0 on success.
+ *
+ */
+int ipc_recv_message(int sockfd, uint32_t message_type,
+                     uint32_t *reply_length, uint8_t **reply);
+
 #endif