Fix klist members that never compiled

rbegin, rend, crbegin and crend were declared returning iterator or
const_iterator while their bodies return reverse_iterator or
const_reverse_iterator -- a hard error the first time any of them is named.

Correcting that exposed the same class of defect one level down: end() const
and cend() build a const_iterator from &_head, which is a const LIST_ENTRY*
in a const member function, while the iterator constructor takes a plain
PLIST_ENTRY. The iterator carries constness in its List template argument
rather than in the entry pointer -- iter_from_entry() const already takes a
non-const PLIST_ENTRY -- so const_cast is the consistent fix.

Verified by explicitly instantiating klist<void(*)()>, which forces every
member to be compiled. Nothing in the tree calls any of these yet.