slirp: update COPYRIGHT to use full 3-Clause BSD License

According to commit 2f5f8996 ("Remove
the advertising clause from the slirp license"), Danny Gasparovski
gave permission to license slirp code under 3-clause BSD license:

    Subject: RE: Slirp license
    Date: Thu, 8 Jan 2009 10:51:00 +1100
    From: "Gasparovski, Daniel" <Daniel.Gasparovski@ato.gov.au>
    To: "Richard Fontana" <rfontana@redhat.com>

    I have no objection to having Slirp code in QEMU be licensed under
    the 3-clause BSD license.

slirp/COPYRIGHT's initial version in 2004 (commit 5fafdf24) listed
only 3 clauses BUT used the poisonous advertising clause for clause 3
which is the controversial clause of non-free 4-clause (that is, it
appears that the BSD-4 license was copied, and then the WRONG clause
was deleted, when creating COPYRIGHT.  Perhaps explained as an easy
mistake to make since 3-clause was created by removing clause 3 of the
4-clause, where you sometimes see the three-clause version with
clauses 1, 2, 4; but more commonly see a renumbered version with
clauses 1, 2, 3 to close the gap. If you pay attention only to clause
numbers instead of content, it can be easy to confuse which clause to
delete to go from 4-clause to 3-clause).

Commit 2f5f8996 removed the poisonous wrong clause on
the grounds of moving from 4-clause to 3-clause; but did not add the
missing clause, which makes it LOOK like the 2-clause version.  But I
think we have a decent enough trail showing the intent for 3-clause.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>

slirp/COPYRIGHT

@@ -25,6 +25,9 @@ The copyright terms and conditions:
  2. Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in the
     documentation and/or other materials provided with the distribution.
+ 3. Neither the name of the copyright holder nor the names of its
+    contributors may be used to endorse or promote products derived
+    from this software without specific prior written permission.
 
  THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
  INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY