systemd: move basic systemd library to shared/nm-utils

For better or worse, we already pull in large parts of systemd sources.

I need a base64 decode implementation (because glib's g_base64_decode()
cannot reject invalid encodings). Instead of coming up with my own or
copy-paste if from somewhere, reuse systemd's unbase64mem().

But for that, make systemd's basic bits an independent static library
first because I will need it in libnm-core.

This doesn't really change anything except making "libnm-systemd-core.la"
an indpendent static library that could be used from "libnm-core". We
shall still be mindful about which internal code of systemd we use, and only
access functionality that is exposed via "systemd/nm-sd-utils-shared.h".

shared/systemd/src/basic/umask-util.h

@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+#include <stdbool.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+
+#include "macro.h"
+
+static inline void umaskp(mode_t *u) {
+        umask(*u);
+}
+
+#define _cleanup_umask_ _cleanup_(umaskp)
+
+struct _umask_struct_ {
+        mode_t mask;
+        bool quit;
+};
+
+static inline void _reset_umask_(struct _umask_struct_ *s) {
+        umask(s->mask);
+};
+
+#define RUN_WITH_UMASK(mask)                                            \
+        for (_cleanup_(_reset_umask_) struct _umask_struct_ _saved_umask_ = { umask(mask), false }; \
+             !_saved_umask_.quit ;                                      \
+             _saved_umask_.quit = true)