areadlink: use SIZE_MAX consistently

areadlink: use SIZE_MAX consistently

* modules/areadlink (Depends-on): Add stdint.
* modules/areadlink-with-size (Depends-on): Likewise.
* lib/areadlink-with-size.c (includes): Drop stdio, since stdlib
gives NULL; drop sys/types, since unistd gives size_t; and add
stdint for SIZE_MAX.
(SIZE_MAX): Rely on headers.
* lib/areadlinkat-with-size.c (includes): Drop stdio, sys/types,
and add stdint.
* lib/areadlink.c (includes): Drop sys/types, and add stdint.
(SIZE_MAX): Likewise.
(INITIAL_BUF_SIZE): Turn into enum.
* lib/areadlinkat.c (INITIAL_BUF_SIZE): Likewise.

Signed-off-by: Eric Blake <ebb9@byu.net>