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/shared/meson.build

@@ -52,10 +52,10 @@ shared_sources = files('''
         import-util.c
         import-util.h
         initreq.h
-        install.c
-        install.h
         install-printf.c
         install-printf.h
+        install.c
+        install.h
         journal-util.c
         journal-util.h
         logs-show.c
@@ -78,6 +78,8 @@ shared_sources = files('''
         resolve-util.c
         resolve-util.h
         seccomp-util.h
+        serialize.c
+        serialize.h
         sleep-config.c
         sleep-config.h
         spawn-ask-password-agent.c
@@ -92,8 +94,8 @@ shared_sources = files('''
         sysctl-util.h
         tomoyo-util.c
         tomoyo-util.h
-        udev-util.h
         udev-util.c
+        udev-util.h
         uid-range.c
         uid-range.h
         utmp-wtmp.h