slirp: clarify license of slirp files using SPDX: explicit BSD

Add SPDX license identifier to clarify the license of files with
explicit 3-clause BSD license header.

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

slirp/src/cksum.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1988, 1992, 1993
  *	The Regents of the University of California.  All rights reserved.

slirp/src/dhcpv6.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * SLIRP stateless DHCPv6
  *

slirp/src/dhcpv6.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Definitions and prototypes for SLIRP stateless DHCPv6
  *

slirp/src/ip.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1993
  *	The Regents of the University of California.  All rights reserved.

slirp/src/ip_icmp.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1988, 1993
  *	The Regents of the University of California.  All rights reserved.

slirp/src/ip_icmp.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1993
  *	The Regents of the University of California.  All rights reserved.

slirp/src/ip_input.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1988, 1993
  *	The Regents of the University of California.  All rights reserved.

slirp/src/ip_output.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1988, 1990, 1993
  *	The Regents of the University of California.  All rights reserved.

slirp/src/mbuf.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1988, 1993
  *	The Regents of the University of California.  All rights reserved.

slirp/src/ncsi-pkt.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright Gavin Shan, IBM Corporation 2016.
  *

slirp/src/ncsi.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * NC-SI (Network Controller Sideband Interface) "echo" model
  *

slirp/src/qtailq.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*      $NetBSD: queue.h,v 1.52 2009/04/20 09:56:08 mschuett Exp $ */
 
 /*

slirp/src/tcp.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1993
  *	The Regents of the University of California.  All rights reserved.

slirp/src/tcp_input.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1988, 1990, 1993, 1994
  *	The Regents of the University of California.  All rights reserved.

slirp/src/tcp_output.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1988, 1990, 1993
  *	The Regents of the University of California.  All rights reserved.

slirp/src/tcp_subr.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1988, 1990, 1993
  *	The Regents of the University of California.  All rights reserved.

slirp/src/tcp_timer.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1988, 1990, 1993
  *	The Regents of the University of California.  All rights reserved.

slirp/src/tcp_timer.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1993
  *	The Regents of the University of California.  All rights reserved.

slirp/src/tcp_var.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1993, 1994
  *	The Regents of the University of California.  All rights reserved.

slirp/src/tcpip.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1993
  *	The Regents of the University of California.  All rights reserved.

slirp/src/udp.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1988, 1990, 1993
  *	The Regents of the University of California.  All rights reserved.

slirp/src/udp.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * Copyright (c) 1982, 1986, 1993
  *	The Regents of the University of California.  All rights reserved.

slirp/src/vmstate.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * VMState interpreter
  *

slirp/src/vmstate.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: BSD-3-Clause */
 /*
  * QEMU migration/snapshot declarations
  *