Switch ufcx.h to the unlicense (#619)

* Switch to unlicense.

* Reformat.

LICENSE

@@ -1,4 +1,5 @@
-The header file ufcx.h is released into the public domain.
+The header file ufcx.h is released using the UNLICENSE. See UNLICENSE for the
+license text.
 
 ------------------------------------------------------------------------------
 

UNLICENSE

@@ -0,0 +1,22 @@
+The software in the file ufcx.h 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 <https://unlicense.org>

ffcx/codegeneration/ufcx.h

@@ -1,5 +1,6 @@
 /// This is UFCx
-/// This code is released into the public domain.
+/// This software is released under the terms of the unlicense (see the file
+/// UNLICENSE).
 ///
 /// The FEniCS Project (http://www.fenicsproject.org/) 2006-2021.
 ///