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/test/test-env-util.c

@@ -2,7 +2,6 @@
 
 #include <string.h>
 
-#include "def.h"
 #include "env-util.h"
 #include "fd-util.h"
 #include "fileio.h"

src/test/test-serialize.c

@@ -1,6 +1,5 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 
-#include "def.h"
 #include "escape.h"
 #include "fd-util.h"
 #include "fileio.h"