Open a file for appending
#include <io.h>
int io_appendfile(int64* d,const char* s);
io_appendfile sets d to the number of a new descriptor writing to the end of the disk file named s, and returns 1. If the file does not exist, it will be created with mode 0600.
If something goes wrong, io_appendfile sets errno to indicate the error, and returns 0; it does not create a new descriptor, and it does not touch d.