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/slice.c

@@ -5,6 +5,7 @@
 #include "alloc-util.h"
 #include "dbus-slice.h"
 #include "log.h"
+#include "serialize.h"
 #include "slice.h"
 #include "special.h"
 #include "string-util.h"
@@ -256,7 +257,8 @@ static int slice_serialize(Unit *u, FILE *f, FDSet *fds) {
         assert(f);
         assert(fds);
 
-        unit_serialize_item(u, f, "state", slice_state_to_string(s->state));
+        (void) serialize_item(f, "state", slice_state_to_string(s->state));
+
         return 0;
 }