update COPYRIGHT file to clarify that permissions apply for all files

these changes are the outcome of a long mailing list thread that took
place March 2016, "musl licensing". among minor other issues,
prospective users were not confident that the whole-project MIT
license would grant permission for files to which the COPYRIGHT file
expressed a belief that copyright not apply, if it turned out that
these files were actually subject to copyright.

in accordance with the original intent of applying a permissive
license to the project, which was that license issues not be an
obstacle to use, the text which was causing confusion is removed. no
new claims of copyright are made, but new text is added to clarify
that the grant of permissions applies to all files, and an explicit
grant of permission to use public headers and crt files without
attribution has been made.

this patch was reviewed and approved by all substantial contributors
to the affected files: Bobby Bingham, John Spencer (rofl0r), Nicholas
J. Kain, Rich Felker, Richard Pennington, Stefan Kristiansson, and
Szabolcs Nagy.

COPYRIGHT

@@ -115,10 +115,8 @@ licensed under following terms: "Permission to use, copy, modify,
 and/or distribute this code for any purpose with or without fee is
 hereby granted. There is no warranty."
 
-The x86_64 port was written by Nicholas J. Kain. Several files (crt)
-were released into the public domain; others are licensed under the
-standard MIT license terms at the top of this file. See individual
-files for their copyright status.
+The x86_64 port was written by Nicholas J. Kain and is licensed under
+the standard MIT terms.
 
 The mips and microblaze ports were originally written by Richard
 Pennington for use in the ellcc project. The original code was adapted
@@ -140,15 +138,26 @@ can be found in the git version control history of the project. The
 omission of copyright and license comments in each file is in the
 interest of source tree size.
 
-All public header files (include/* and arch/*/bits/*) should be
-treated as Public Domain as they intentionally contain no content
-which can be covered by copyright. Some source modules may fall in
-this category as well. If you believe that a file is so trivial that
-it should be in the Public Domain, please contact the authors and
-request an explicit statement releasing it from copyright.
+In addition, permission is hereby granted for all public header files
+(include/* and arch/*/bits/*) and crt files intended to be linked into
+applications (crt/*, ldso/dlstart.c, and arch/*/crt_arch.h) to omit
+the copyright notice and permission notice otherwise required by the
+license, and to use these files without any requirement of
+attribution. These files include substantial contributions from:
 
-The following files are trivial, believed not to be copyrightable in
-the first place, and hereby explicitly released to the Public Domain:
+Bobby Bingham
+John Spencer
+Nicholas J. Kain
+Rich Felker
+Richard Pennington
+Stefan Kristiansson
+Szabolcs Nagy
+
+all of whom have explicitly granted such permission.
 
-All public headers: include/*, arch/*/bits/*
-Startup files: crt/*
+This file previously contained text expressing a belief that most of
+the files covered by the above exception were sufficiently trivial not
+to be subject to copyright, resulting in confusion over whether it
+negated the permissions granted in the license. In the spirit of
+permissive licensing, and of not having licensing issues being an
+obstacle to adoption, that text has been removed.