Open a file for reading and writing
#include <io.h>
int io_readwritefile(int64* d,const char* s);
io_readwritefile sets d to the number of a new descriptor reading from and writing to the disk file named s, and returns 1. The file needs to exist.
If something goes wrong, io_readwritefile sets errno to indicate the error, and returns 0; it does not create a new descriptor, and it does not touch d.