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>
@harshavardhana This is -1 when cast to an int. So, the system call is returning a -1 which when stored in a uintptr is ^uintptr(0). IMO, I think we can rid the check as its behaviour is like any other system call returning -1 on failure. 
rc := ^uintptr((0))
fmt.Println(int(rc)) // will print -1