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.

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