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/env-util.h

@@ -45,6 +45,3 @@ char *strv_env_get(char **x, const char *n) _pure_;
 
 int getenv_bool(const char *p);
 int getenv_bool_secure(const char *p);
-
-int serialize_environment(FILE *f, char **environment);
-int deserialize_environment(char ***environment, const char *line);