Simplify KWSys per-source license notices

Per-source copyright/license notice headers that spell out copyright holder
names and years are hard to maintain and often out-of-date or plain wrong.
Precise contributor information is already maintained automatically by the
version control tool.  Ultimately it is the receiver of a file who is
responsible for determining its licensing status, and per-source notices are
merely a convenience.  Therefore it is simpler and more accurate for
each source to have a generic notice of the license name and references to
more detailed information on copyright holders and full license terms.

Our `Copyright.txt` file now contains a list of Contributors whose names
appeared source-level copyright notices.  It also references version control
history for more precise information.  Therefore we no longer need to spell
out the list of Contributors in each source file notice.

Replace KWSys per-source copyright/license notice headers with a short
description of the license and links to `Copyright.txt` and online
information available from "https://cmake.org/licensing#kwsys".

Run the `filter-notices.bash` script to perform the replacements mechanically.
Manually fix up the shebang line in one file.

Change-Id: I8497f7c868664dcf54a8608ab302ad93c860b334