This causes a segmentation fault...

This way, we don't have to pass the full "Options" object to
daemonize().

The constructor/destructor functions parse and free the configuration
properly.  This way, we don't have to load the pid file path more than
once.

Moved changeToUser(), cleanUpPidFile(), killFromPidFile() to
daemon.c.  These are daemonization functions.

Renamed functions, types, variables.

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.