Fix Deprecation Warnings

The project contains various deprecation warnings which are easy to
fix:

 * replace `Library` with `LibraryLoader`
 * replace `com.kenai.constantine.platform.*` with
   `jnr.constants.platform.*`
 * make `public` methods in `SockAddrUnix` package protected since the
   class and all of its subclasses are package protected

The changes should all be backwards compatible since:

 * `SockAddrUnix` and all of its subclasses are package protected
 * `SockAddrUnix` is package protected