Added support for RFC 959's REST command (restart), closing SF patch

Added support for RFC 959's REST command (restart), closing SF patch
#101187, which some modifications.  Specifically,

ntransfercmd(), transfercmd(), and retrbinary() all grow an optional
`rest' argument, which if not None, is used as the argument to an FTP
REST comman dbefore the socket is returned.  Differences from the SF
patch:

- always compare against None with `is' or `is not' instead of == or !=

- no parens around conditional

- RFC 959 defines the argument to REST is a string containing any
  ASCII characters in the range [33..126].  Therefore, we use the %s
  format character instead of %f or %d as suggested in the patch's
  comments.  Note that we do /not/ sanity checkthe contents of the
  rest argument (but we'll document this in the library reference
  manual).