macOS qrcodes, shred and hdiutil

[ritewhose]

Hi all,

1) QR code handling for macOS (the reason I'm here):
    - Preview.app ships with macOS and is basically immovable. Everyone has it.
    - imagemagick/graphicsmagick need xquartz installed, and at least through homebrew the programs need to be re recompiled with X11 support.
So I think Preview.app is a better choice. It's easiest and does the same thing.

2) remove unused shred option and hdid>hdiutil
Since I had to read the code anyway I tacked these on too. I think we can remove "SHRED=srm ... " (deprecated, unused due to ramdisk) and change "hdid" (deprecated as of 10.3) to "hdiutil attach". Anyway, the code works as-is, so it's up to you.

-rw