Update system/xattrs_linux.go

- Add Llistxattr() support
- Additionally cleanup Lgetxattr() and implement it properly
  in accordance with getxattr() syscall behavior.

Signed-off-by: Harshavardhana <fharshav@redhat.com>
all platforms to support extended attributes, removing windows from the picture.  ENOTSUP is the same errno across all platforms. 
But if you see "// +build linux" is added to be specific for Linux. 

is libcontainer slated to available on multiple platforms? as in non-Linux?