Use system() to determine clipboard on linux and redirect stderr.

Hey,

here are a few changes. The main reason was to get rid of the error messages of the which command. I am working on a commandline tool, so they are pretty ugly.

- system() returns boolean, that's what we need here
- redirect stdout and stderr to /dev/null to avoid ugly error messages
- set file mode 0600 in file implementation, so only you can access your clipboard