ImageShow improvements

[radarhere]

• Added UnixViewer implementation of the get_command method
• Supply filename through stdin instead of inline in Mac and Unix viewers. This is for the sake of security - rather than simply being quoted through shlex/pipes, the filename is now a variable. Feel free to tell me that this is more convoluted than necessary though, given that the filename being passed in does not originate externally.

[abarker]

The evaluated shell variables $im need to be quoted or problems can arise with temp pathnames that have spaces in them.

#!/bin/bash

touch bar
mkdir "foo bar zoo"

TMP="foo bar zoo" python3 - <<'____HERE'
from PIL import Image
im = Image.open(r"/var/lib/app-info/icons/ubuntu-focal-updates-universe/48x48/inkscape_inkscape.png")
im.show()
____HERE

stat bar # The file is now deleted!

[radarhere]

That problem should now be fixed, thanks to #6010, and released in Pillow 9.0.1.