(PUP-837) FFI Puppet::Util::Windows::SID

 - FFI Win32 API functions IsValidSid, ConvertSidToStringSid,
   ConvertStringSidToSid and LocalFree
 - Unfortunately ConvertSidToStringSidW allocates a buffer and does not
   return it's length, therefore a new helper function has been added to
   FFI::Pointer that will return a string up to a maximum length. If the
   NULL terminator is encountered prior to the max size, no additional
   memory is read.  This is not ideal, but slightly more secure than the
   previous implementation, which picked a buffer size, read all memory
   to the buffer, then called .strip on what remained.  The helper
   function also imposes a maximum of 512 UTF16 characters (1024 bytes)
 - Presume a maximum SID string length of 184 characters when formatted
   per http://stackoverflow.com/a/1792930
 - Add additional FFI MemoryPointer aliases for :wchar, #read_wchar
 - Remove any dependenc on mixins Windows::Security, Windows::Memory or
   Windows::MSVCRT
 - Updated affected callsites in security.rb to properly use or create
   FFI::Pointer when dealing with IsValidSid and sid_ptr_to_string. Also
   updated a couple of existing "old school" calls that are expecting
   addresses instead of FFI::Pointer objects.  This is a partial
   refactor until work continues with making security FFI compatible.
(maint) Windows SID variable naming style

This is a minor change to be more specific with the naming of the variable
'string' to 'string_sid' for methods in Puppet::Util::Windows::SID.