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/basic/fileio.h

@@ -10,6 +10,8 @@
 #include "macro.h"
 #include "time-util.h"
 
+#define LONG_LINE_MAX (1U*1024U*1024U)
+
 typedef enum {
         WRITE_STRING_FILE_CREATE            = 1 << 0,
         WRITE_STRING_FILE_ATOMIC            = 1 << 1,