util-lib: move open_serialization_fd() to serialize.c

It definitely fits better there.

No code changes, just some rearranging.

src/shared/serialize.c

@@ -1,12 +1,16 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 
+#include <sys/mman.h>
+
 #include "alloc-util.h"
 #include "env-util.h"
 #include "escape.h"
 #include "fileio.h"
 #include "parse-util.h"
+#include "process-util.h"
 #include "serialize.h"
 #include "strv.h"
+#include "tmpfile-util.h"
 
 int serialize_item(FILE *f, const char *key, const char *value) {
         assert(f);
@@ -188,3 +192,22 @@ int deserialize_environment(const char *value, char ***list) {
         unescaped = NULL; /* now part of 'list' */
         return 0;
 }
+
+int open_serialization_fd(const char *ident) {
+        int fd;
+
+        fd = memfd_create(ident, MFD_CLOEXEC);
+        if (fd < 0) {
+                const char *path;
+
+                path = getpid_cached() == 1 ? "/run/systemd" : "/tmp";
+                fd = open_tmpfile_unlinkable(path, O_RDWR|O_CLOEXEC);
+                if (fd < 0)
+                        return fd;
+
+                log_debug("Serializing %s to %s.", ident, path);
+        } else
+                log_debug("Serializing %s to memfd.", ident);
+
+        return fd;
+}