Solve the problem with macOS sandboxing

[ilmaisin]

Following was told me in IRC:

16:00 -!- Irssi: Starting query in Freenode with nick omitted
16:00 <nick omitted> hi, are you working on cups-mailto? :)
16:01 <nick omitted> in its current state it's impossible to get it to work with os x el capitan and newer
16:02 <nick omitted> because they took away the sandbox: off option in cups-files.conf
16:03 <nick omitted> i'm not an os x user myself though, so it's a bit difficult to debug