headers: add spdx tags to imported files with a known license

I added the header in the cases where the license text is present and it is
easy to find the appropriate SPDX header.

For "public domain" stuff: SDPX treats each "public domain" license as unique [1],
but luckily the one in siphash24.c is one of the identified variants. There are
some other cases which specify "public domain" but there doesn't seem to be a
SPDX identifier.

[1] https://wiki.spdx.org/view/Legal_Team/Decisions/Dealing_with_Public_Domain_within_SPDX_Files

gunicode.[ch] are imported from glib, which is licensed as LGPL2.1+,
see https://gitlab.gnome.org/GNOME/glib/-/blob/master/glib/gunicode.h.

src/basic/gunicode.c

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /* gunicode.c - Unicode manipulation functions
  *
  *  Copyright (C) 1999, 2000 Tom Tromey

src/basic/gunicode.h

@@ -1,10 +1,10 @@
-#pragma once
-
+/* SPDX-License-Identifier: LGPL-2.1+ */
 /* gunicode.h - Unicode manipulation functions
  *
  *  Copyright (C) 1999, 2000 Tom Tromey
  *  Copyright © 2000, 2005 Red Hat, Inc.
  */
+#pragma once
 
 #include <stdbool.h>
 #include <stdint.h>

src/basic/siphash24.c

@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: CC0-1.0 */
+
 /*
    SipHash reference C implementation
 
@@ -10,7 +12,7 @@
    worldwide. This software is distributed without any warranty.
 
    You should have received a copy of the CC0 Public Domain Dedication along with
-   this software. If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.
+   this software. If not, see <https://creativecommons.org/publicdomain/zero/1.0/>.
 
    (Minimal changes made by Lennart Poettering, to make clean for inclusion in systemd)
    (Refactored by Tom Gundersen to split up in several functions and follow systemd

src/basic/siphash24.h

@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: CC0-1.0 */
+
 #pragma once
 
 #include <inttypes.h>

src/boot/efi/sha256.c

@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
 /* Stolen from glibc and converted to UEFI style. In glibc it comes with the following copyright blurb: */
 
 /* Functions to compute SHA256 message digest of files or memory blocks.

src/shared/initreq.h

@@ -1,3 +1,4 @@
+/* SPDX-License-Identifier: LGPL-2+ */
 /*
  * initreq.h	Interface to talk to init through /dev/initctl.
  *

src/shared/linux/nl80211.h

@@ -1,3 +1,5 @@
+/* SPDX-License-Identifier: MIT */
+
 #ifndef __LINUX_NL80211_H
 #define __LINUX_NL80211_H
 /*