Improvements to chsmack

[@jobol: can you briefly enumerate/summarize improvements here]

Here is the list of the improvements:
* use of linux header for the name of the attributes
* validation of the validity of the labels
* use of the library libsmack where possible
* preparing to use future versions of libsmack
* new option to follow symbolic links
* new option to remove smack labels
* better validation of options and arguments
* refusing to set transmute if not a directory
* updated/improved manual page