Fix permissions on album art files

Hi,
I noticed that album art files were written with incorrect (in my case) permissions. After some looking at the code, it seems that the daemonize-option sets umask to 0077, which is then used as well when writing album art and torrent files in the blackhole directory. This patch saves the original umask in a global configuration variable, which is then used when writing files which need to be world-readable, such as album art.

Regards,

Michel.