Move LONG_LINE_MAX definition to fileio.h

All users of the macro (except for one, in serialize.c), use the macro in
connection with read_line(), so they must include fileio.h.  Let's not play
libc games and require multiple header file to be included for the most common
use of a function.

The removal of def.h includes is not exact. I mostly went over the commits that
switch over to use read_line() and add def.h at the same time and reverted the
addition of def.h in those files.

src/shared/serialize.c

@@ -1,9 +1,9 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 
 #include "alloc-util.h"
-#include "def.h"
 #include "env-util.h"
 #include "escape.h"
+#include "fileio.h"
 #include "parse-util.h"
 #include "serialize.h"
 #include "strv.h"