BSD-2-Clause: Make "All rights reserved" optional

The OSI version, which we list as the upstream, does not include "All
rights reserved":

  $ curl -s https://opensource.org/licenses/BSD-2-Clause | grep -A2 Copyright
      <p>Copyright &lt;YEAR&gt; &lt;COPYRIGHT HOLDER&gt;<br /></p>

      <p>Redistribution and use in source and binary forms...

Also:

* Remove a pre-<year> space from within the <alt>.  There's leading
  space before the <alt>, so we don't need the post-<alt> space.
* Add a period which was missing before.  Without the period, there's
  no terminal punctuation before we start the "All rights reserved"
  sentence.

src/BSD-2-Clause.xml

@@ -7,7 +7,7 @@
       </crossRefs>
       <copyrightText>
          <p>Copyright (c) 
-        <alt match=".+" name="copyright"> &lt;year&gt; &lt;owner&gt;</alt> All rights reserved. 
+        <alt match=".+" name="copyright">&lt;year&gt; &lt;owner&gt;</alt><optional>. All rights reserved.</optional>
       </p>
       </copyrightText>