Assure that attribute name starts with a valid prefix.

Took me 3 hours to figure out that I needed to prefix my xattr -w 'attribute name' with something like "user," "system," "trusted," "security." This issue came up in my Ubuntu 14.04, but I had no problem with Mac OSX 10.9.

I'd like to propose that for consistency's sake (across platforms), prefixes should be checked. Otherwise, it could be more helpful to produce a more descriptive error message. The system typically returns an "[Errno 95] This operation is not supported."