copyright: update the years, use ranges, and explain this usage

The interval 2013-2017 for the Free Software Foundation is valid
because in those years there were releases with changes by either
Chris or David, and the GNU maintainers guide advises to mention
a new year in all files of a package, not just in the ones that
actually changed, and be done with it for the rest of the year.

src/utils.c

 /**************************************************************************
  *   utils.c  --  This file is part of GNU nano.                          *
  *                                                                        *
- *   Copyright (C) 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007,  *
- *   2008, 2009, 2010, 2011, 2013, 2014 Free Software Foundation, Inc.    *
+ *   Copyright (C) 1999-2011, 2013-2017 Free Software Foundation, Inc.    *
  *   Copyright (C) 2016 Benno Schulenberg                                 *
  *                                                                        *
  *   GNU nano is free software: you can redistribute it and/or modify     *

src/winio.c

 /**************************************************************************
  *   winio.c  --  This file is part of GNU nano.                          *
  *                                                                        *
- *   Copyright (C) 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007,  *
- *   2008, 2009, 2010, 2011, 2013, 2014 Free Software Foundation, Inc.    *
+ *   Copyright (C) 1999-2011, 2013-2017 Free Software Foundation, Inc.    *
  *   Copyright (C) 2014, 2015, 2016, 2017 Benno Schulenberg               *
  *                                                                        *
  *   GNU nano is free software: you can redistribute it and/or modify     *