Change to a CC0-based license. (#3831)

* Change to a CC0-based license.

* I always forget the link syntax.

LICENSE.md

+Unless otherwise noted, this project is in the public domain in the
+United States.
+
+It contains materials that originally came from the United States
+Geological Survey, an agency of the United States Department of
+Interior.  For more information on their copyright policies, see
+https://www.usgs.gov/information-policies-and-instructions/copyrights-and-credits
+
+It also contains materials from contributors that have waived their
+copyright interest to the public domain.
+
+Additionally, the authors waive copyright and related rights in the
+work worldwide through the CC0 1.0 Universal public domain dedication.
+
+CC0 1.0 Universal Summary
+-------------------------
+
+This is a human-readable summary of the [Legal Code (read the full
+text)](https://creativecommons.org/publicdomain/zero/1.0/legalcode).
+
+
+### No Copyright
+
+The authors have associated their contributions to the ISIS project
+with this deed, and have dedicated the work to the public domain
+by waiving all of their rights to the work worldwide under copyright
+law, including all related and neighboring rights, to the extent
+allowed by law.
+
+You can copy, modify, distribute and perform the work, even for
+commercial purposes, all without asking permission.
+
+
+### Other Information
+
+In no way are the patent or trademark rights of any person affected
+by CC0, nor are the rights that other persons may have in the work
+or in how the work is used, such as publicity or privacy rights.
+
+Unless expressly stated otherwise, the authors who have associated
+the ISIS project with this deed make no warranties about the work,
+and disclaim liability for all uses of the work, to the fullest
+extent permitted by applicable law. When using or citing the work,
+you should not imply endorsement by the authors.

LICENSE.txt

-This is free and unencumbered software released into the public domain.
-
-Anyone is free to copy, modify, publish, use, compile, sell, or
-distribute this software, either in source code form or as a compiled
-binary, for any purpose, commercial or non-commercial, and by any
-means.
-
-In jurisdictions that recognize copyright laws, the author or authors
-of this software dedicate any and all copyright interest in the
-software to the public domain. We make this dedication for the benefit
-of the public at large and to the detriment of our heirs and
-successors. We intend this dedication to be an overt act of
-relinquishment in perpetuity of all present and future rights to this
-software under copyright law.
-
-THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
-EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
-MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
-IN NO EVENT SHALL THE AUTHORS BE LIABLE FOR ANY CLAIM, DAMAGES OR
-OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
-ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
-OTHER DEALINGS IN THE SOFTWARE.
-
-For more information, please refer to <http://unlicense.org/>