rewrite syscall wrappers

Darwin:
- replace syscall.StringBytePtr by syscall.BytePtrFromString.
  syscall.StringBytePtr is deprecated, because it panics on wrong input.
- don't hide "options" flag in (get|set|list|remove)xattr(2). It will be
  used in functions for symbolic links: lget(), lset(), etc.
- don't hide "position" flag in setxattr(2) and getxattr(2).

Linux:
- reuse syscall.(Get|Set|List|Remove)xattr wrappers.

Add get(), set(), list() and remove() generic functions which are
identical on all platforms and used by high-level API.
add GetTo: efficient variant of Get
README: update links to this repo

Remove nut.json.
remove dependency from gocheck