www/url_handlers.c: Use a capabilities variable in admin_list_users()

In admin_list_users() we were doing a lot of

    if (atoi(get_var(db_row, "capabilities")) & A_CAPABILITY)

these all now become

    if (capabilities & A_CAPABILITY)

Much nicer.

Signed-off-by: Andrew Clayton <andrew@opentechlabs.co.uk>