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/def.h

@@ -74,6 +74,4 @@
                 "/usr/lib/" n                   \
                 _CONF_PATHS_SPLIT_USR(n))
 
-#define LONG_LINE_MAX (1U*1024U*1024U)
-
 #define HIGH_RLIMIT_MEMLOCK (1024ULL*1024ULL*64ULL)