Extended attribute support for Go (linux + darwin + freebsd).
"Extended attributes are name:value pairs associated permanently with files and directories, similar to the environment strings associated with a process. An attribute may be defined or undefined. If it is defined, its value may be empty or non-empty." [See more...](https://en.wikipedia.org/wiki/Extended_file_attributes)
`SetWithFlags` allows to additionally pass system flags to be forwarded to the underlying calls, FreeBSD does not support this and the parameter will be ignored.
The `L` variants of all functions (`LGet/LSet/...`) are identical to `Get/Set/...` except that they
do not reference a symlink that appears at the end of a path. See
[GoDoc](http://godoc.org/github.com/pkg/xattr) for details.