class path funcs taking iterators making poor assumption

[Beman Dawes]

class path funcs taking iterators are assuming the value_type of the iterator is a character type. But it is perfectly natural to write something like path p(q.begin(), q.end()) where q is a path or a vector of strings, so the value type is a container.

The fix is to add several overloads depending on the value_type of the iterator.

The rule-of-thumb might be something like "if the operation has an overload of a single item of type T, it should also handle iteration over a sequence of T's.

[Beman Dawes]

The motivation for this change is not strong enough to justify deviating from the ISO File System Technical Specification.

--Beman