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/basic/time-util.h

@@ -108,10 +108,6 @@ char *format_timestamp_us_utc(char *buf, size_t l, usec_t t);
 char *format_timestamp_relative(char *buf, size_t l, usec_t t);
 char *format_timespan(char *buf, size_t l, usec_t t, usec_t accuracy);
 
-void dual_timestamp_serialize(FILE *f, const char *name, dual_timestamp *t);
-int dual_timestamp_deserialize(const char *value, dual_timestamp *t);
-int timestamp_deserialize(const char *value, usec_t *timestamp);
-
 int parse_timestamp(const char *t, usec_t *usec);
 
 int parse_sec(const char *t, usec_t *usec);