Move code from Path::RelativeFS() and make it generic.

Reduce overhead, don't duplicate the string.

There is no GetLengthUTF8() because strlen or std::string::size()
could be used instead.

- Move definition of SEPARATOR_UTF8 out of #ifdef
- Remove duplicated check in IsSeparatorUTF8

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.