Fix copy-file on MS-Windows with file names outside of current locale.

Fix copy-file on MS-Windows with file names outside of current locale.

 src/fileio.c (Fcopy_file) [WINDOWSNT]: Move most of the
 Windows-specific code to w32.c.  Change error message text to
 match that of Posix platforms.
 src/w32.c (w32_copy_file): New function, most of the code copied and
 reworked from Fcopy_file.  Improve error handling.  Plug memory
 leak when errors are thrown.  Support file names outside of the
 current codepage.  (Bug#7100)