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/path.c

@@ -14,6 +14,7 @@
 #include "macro.h"
 #include "mkdir.h"
 #include "path.h"
+#include "serialize.h"
 #include "special.h"
 #include "stat-util.h"
 #include "string-table.h"
@@ -600,8 +601,8 @@ static int path_serialize(Unit *u, FILE *f, FDSet *fds) {
         assert(f);
         assert(fds);
 
-        unit_serialize_item(u, f, "state", path_state_to_string(p->state));
-        unit_serialize_item(u, f, "result", path_result_to_string(p->result));
+        (void) serialize_item(f, "state", path_state_to_string(p->state));
+        (void) serialize_item(f, "result", path_result_to_string(p->result));
 
         return 0;
 }