Introduce a separate TU for utility functions util.c. Add a function
exec_cmd to simplify execution of system commands with error handling.
While at it, suppress a warning about unused result when executing a
shell command. As we only display the command's output we do not care
about the exit code.
This fixes Debian bug #398989[0]
[0]: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=398989
