implicitly declare list elements in ISC_LIST_FOREACH macros

implicitly declare list elements in ISC_LIST_FOREACH macros

ISC_LIST_FOREACH and related macros now use 'typeof(list.head)' to
declare the list elements automatically; the caller no longer needs
to do so.

ISC_LIST_FOREACH_SAFE also now implicitly declares its own 'next'
pointer, so it only needs three parameters instead of four.