Switching the Cygwin DLL to LGPLv3+, dropping commercial buyout option

Bump GPLv2+ to GPLv3+ for some files, clarify BSD 2-clause.

Everything else stays under GPLv3+.

New Linking Exception exempts resulting executables from LGPLv3 section 4.

Add CONTRIBUTORS file to keep track of licensing.

Remove 'Copyright Red Hat Inc' comments.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/utils/ssp.c

@@ -1,5 +1,4 @@
 /*
- * Copyright (c) 2000, 2001, 2002, 2009, 2011, 2013 Red Hat, Inc.
  *
  *     This program is free software; you can redistribute it and/or modify
  *     it under the terms of the GNU General Public License as published by
@@ -843,7 +842,7 @@ print_version ()
 {
   printf ("ssp (cygwin) %d.%d.%d\n"
 	  "Single-Step Profiler\n"
-	  "Copyright (C) 2000 - %s Red Hat, Inc.\n"
+	  "Copyright (C) 2000 - %s Cygwin Authors\n"
 	  "This is free software; see the source for copying conditions.  There is NO\n"
 	  "warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.\n",
 	  CYGWIN_VERSION_DLL_MAJOR / 1000,