* dtable.cc (dtable::build_fhandler): Accept an optional path_conv argument.

* dtable.cc (dtable::build_fhandler): Accept an optional path_conv argument.
If available, use this to calculate path name and device number.
* dtable.h (dtable): Reflect above change.
* fhandler.h (fhandler_base): Declare virtual method which accepts path_conv
rather than path string as first argument.
* fhandler.cc (fhandler_base::open): Define above new method.
* syscalls.cc (_open): Set aside a path_conv variable for use in build_fhandler
and subsequent call to open.