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/setfacl.c

@@ -1,8 +1,5 @@
 /* setfacl.c
 
-   Copyright 2000, 2001, 2002, 2003, 2006, 2008, 2009, 2010, 2011, 2014,
-   2015 Red Hat Inc.
-
    Written by Corinna Vinschen <vinschen@redhat.com>
 
 This file is part of Cygwin.
@@ -674,7 +671,7 @@ print_version ()
 {
   printf ("setfacl (cygwin) %d.%d.%d\n"
 	  "POSIX ACL modification utility\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,