write_file() - appends a string to a file
int write_file( string file, string str, int flag );
Append the string 'str' into the file 'file'. Returns 0 or 1 for fail‐
ure or success. If flag is 1, write_file overwrites instead of append‐
ing.
file_size(3), read_file(3), write_buffer(3), async_read(3), async_write(3),
valid_read(4), valid_write(4)