Fix win64 portability warning

Reported by Ozkan Sezer <sezeroz@gmail.com>:

There are also warnings wherever list_entry() macro is used, such as:

src/load.c: In function 'unlink_tempfiles':
src/load.c:404:8: warning: cast from pointer to integer of different size

... or:

options.c: In function 'usage':
options.c:52:8: warning: cast from pointer to integer of different size
options.c:57:8: warning: cast from pointer to integer of different size

... which may be cured by changing (unsigned long) cast with (size_t)

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Update changelog

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>