(PUP-391) Use FFI.errno instead of GetLastError

 - In commit bf6f5c4e67c527d97da2dd3ee692692c8b9a8981, @@GetLastError
   was introduced as a band-aid to ensure the appropriate error code
   was read out of the thread local storage after a Win32 API call was
   made.  At that time, not all of the code had been converted to FFI
   compatible and not all gems had been upgraded to FFI compatible.
   Calls still made using Win32API were not captured by FFI.errno (which
   only calls GetLastError *after* making an FFI defined Win32 API call)
   resulting in a number of test fails.  The solution at the time was
   to manually call GetLastError instead to retrieve the value set by
   SetLastError.
 - Now that all Win32 API calls are made through FFI, FFI will
   automatically capture API results by calling GetLastError, and we
   can remove the unnecessary binding to GetLastError here.
(PUP-391) Fix Rdoc 4 specs on Windows Ruby 2

 - When calling the document method on RDoc and the root option has not
   been specified, RDoc will provide a default in the options class:
   @root = Pathname(Dir.pwd).  NOTE: This does not affect RDoc 3.9.5
   that ships with Ruby 1.9.3 on Windows, only RDoc 4.0.0+ that ships
   with Ruby 2.0.0+ on Windows.
 - The win32-dir monkey patches the Dir.pwd method, and instead of
   using / separators like the rest of Ruby, it uses \ as a path
   separator.
 - RDoc uses Pathname, specifically the relative_path_from method
   which expects / separators, and gets confused with \.  Therefore,
   provide a workaround to ensure that Windows uses the appropriate
   / in paths.
(PUP-391) Ruby 2 exception type changed to EISDIR

 - File.open now raises EISDIR when a path is a directory, prior to
   verifying permissions in Ruby 2.  Files continue to return EACCESS.