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/core/dynamic-user.c

@@ -12,6 +12,7 @@
 #include "io-util.h"
 #include "parse-util.h"
 #include "random-util.h"
+#include "serialize.h"
 #include "socket-util.h"
 #include "stdio-util.h"
 #include "string-util.h"
@@ -607,13 +608,13 @@ int dynamic_user_serialize(Manager *m, FILE *f, FDSet *fds) {
 
                 copy0 = fdset_put_dup(fds, d->storage_socket[0]);
                 if (copy0 < 0)
-                        return copy0;
+                        return log_error_errno(copy0, "Failed to add dynamic user storage fd to serialization: %m");
 
                 copy1 = fdset_put_dup(fds, d->storage_socket[1]);
                 if (copy1 < 0)
-                        return copy1;
+                        return log_error_errno(copy1, "Failed to add dynamic user storage fd to serialization: %m");
 
-                fprintf(f, "dynamic-user=%s %i %i\n", d->name, copy0, copy1);
+                (void) serialize_item_format(f, "dynamic-user", "%s %i %i", d->name, copy0, copy1);
         }
 
         return 0;