core: rework serialization

Let's be more careful with what we serialize: let's ensure we never
serialize strings that are longer than LONG_LINE_MAX, so that we know we
can read them back with read_line(…, LONG_LINE_MAX, …) safely.

In order to implement this all serialization functions are move to
serialize.[ch], and internally will do line size checks. We'd rather
skip a serialization line (with a loud warning) than write an overly
long line out. Of course, this is just a second level protection, after
all the data we serialize shouldn't be this long in the first place.

While we are at it also clean up logging: while serializing make sure to
always log about errors immediately. Also, (void)ify all calls we don't
expect errors in (or catch errors as part of the general
fflush_and_check() at the end.

src/shared/serialize.h

@@ -0,0 +1,23 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+#pragma once
+
+#include <stdio.h>
+
+#include "fdset.h"
+#include "macro.h"
+
+int serialize_item(FILE *f, const char *key, const char *value);
+int serialize_item_escaped(FILE *f, const char *key, const char *value);
+int serialize_item_format(FILE *f, const char *key, const char *value, ...) _printf_(3,4);
+int serialize_fd(FILE *f, FDSet *fds, const char *key, int fd);
+int serialize_usec(FILE *f, const char *key, usec_t usec);
+int serialize_dual_timestamp(FILE *f, const char *key, const dual_timestamp *t);
+int serialize_strv(FILE *f, const char *key, char **l);
+
+static inline int serialize_bool(FILE *f, const char *key, bool b) {
+        return serialize_item(f, key, yes_no(b));
+}
+
+int deserialize_usec(const char *value, usec_t *timestamp);
+int deserialize_dual_timestamp(const char *value, dual_timestamp *t);
+int deserialize_environment(const char *value, char ***environment);