There is only one valid error condition for setsid(): when the current
process is already the process group leader.  This is non-critical.

To detach from the parent process, fork once and make the old process
exit.  No need to do that twice.

Don't bother to call fstat() or isatty() on STDIN_FILENO.

redirect_stdin() is a daemonization function, and disconnecting from
the standard input is always a good idea for MPD.