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.

If fs_charset is empty, i.e. we're using the default "utf-8",
GetFSCharset() should return exactly that instead of an empty
std::string.

Implement a fast path for UTF-8 which leaves fs_charset empty, and
don't assign a value to fs_charset if there's no configuration.