no whatis in Arch linux

When there's no whatis on the system, this part fails, and dex as well.
```
				if os.name == 'posix':
					whatis = subprocess.Popen(['whatis', filename], stdout=subprocess.PIPE, stderr=subprocess.PIPE)
					stdout, stderr = whatis.communicate()
					res = stdout.decode(sys.stdin.encoding).split('- ', 1)
					if len(res) == 2:
						de.set_value('Comment', res[1].split(os.linesep, 1)[0])
```