Unreliable determining seekability in ogg123

In vorbis-tools/ogg123/file_transport.c, the input is considered not to be seekable if "-" (stdin) is specified as input file.

In Unix, you can actually seek in stdin if it comes from a regular file (instead of from a pipe).

Seekable stdin: ogg123 - < file.ogg

Non-seekable standard-specified file: mkfifo fifo.ogg ogg123 fifo.ogg

I propose to correctly determine the seekability by stat()ing the descriptor.

bool isSeekable(int fd) {
    struct stat sb;
    int res = fstat(fd, &sb);
    if (res < 0) {
        perror("fstat");
        return -1;
    }
     return S_ISREG(sb.st_mode) !=0;
}

and then doing something like

private->seekable = isSeekable(fileno(private->fp));