open_append - open a file for appending
int open_append(const char *filename);
open_append opens the file filename for appending write-only use and
returns the file handle. If it does not exist, it will be created with mode
0600. If there was an error opening or creating the file, open_append returns
-1 and sets errno accordingly.
All write operations will append after the last byte, regardless
of previous calls to lseek(2) or other processes also appending to the same