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/machine/machine.c

@@ -21,6 +21,7 @@
 #include "mkdir.h"
 #include "parse-util.h"
 #include "process-util.h"
+#include "serialize.h"
 #include "special.h"
 #include "stdio-util.h"
 #include "string-table.h"
@@ -284,9 +285,9 @@ int machine_load(Machine *m) {
         }
 
         if (realtime)
-                timestamp_deserialize(realtime, &m->timestamp.realtime);
+                (void) deserialize_usec(realtime, &m->timestamp.realtime);
         if (monotonic)
-                timestamp_deserialize(monotonic, &m->timestamp.monotonic);
+                (void) deserialize_usec(monotonic, &m->timestamp.monotonic);
 
         if (netif) {
                 size_t allocated = 0, nr = 0;