Rearranged the project to have src/ and build/ directories

This simplifies keeping everything clean.

src/readwrite.h

@@ -0,0 +1,11 @@
+#ifndef __READWRITE_H
+
+#define __READWRITE_H
+
+int socket_connect(struct sockaddr* to);
+off64_t socket_nbd_read_hello(int fd);
+void socket_nbd_read(int fd, off64_t from, int len, int out_fd, void* out_buf);
+void socket_nbd_write(int fd, off64_t from, int len, int out_fd, void* out_buf);
+
+#endif
+