Add "path.h" include throughout, where needed. Use new path_conv methods and

Add "path.h" include throughout, where needed.  Use new path_conv methods and
operators to simplify testing for directory and attributes, throughout.
* path.h (path_conv::exists): New method.
(path_conv::has_attribute): Ditto.
(path_conv::isdir): Ditto.
(path_conv::DWORD &): New operator.
(path_conv::int &): Ditto.
* dir.cc (rmdir): Eliminate a goto.
* dtable.cc (dtable::build_fhandler): Accept opt and suffix info for
path_conv.check.  Return fh == NULL on path_conv error.  Pass unit to set_name
as appropriate.
(dtable::reset_unix_path_name): New method.
* dtable.h (dtable): Declare new method.  Reflect arg changes to
build_fhandler.
* fhandler.cc (fhandler_disk_dummy_name): Eliminate.
(fhandler_base::set_name): Expect paths to be NULL.  Build unix_path_name from
win32_path_name when it is a device.
(fhandler_base::reset_unix_path_name): New method.
(fhandler_base::raw_read): Report EISDIR when ERROR_INVALID_FUNCTION or
ERROR_INVALID_PARAMETER and reading a directory.
(fhandler_disk_file::fstat): Don't call stat_dev since we should now never be
calling fhandler_disk_file methods with devices.
(fhandler_base::fhandler_base): Clear {unix,win32}_path_name.
(fhandler_base::~fhandler_base): Always free {unix,win32}_path_name.
(fhandler_disk_file::fhandler_disk_file): Remove set_no_free_names kludge.
(fhandler_disk_file::open): Ditto.
* fhandler.h (fhandler_base::no_free_names): Eliminate.
(fhandler_base::set_no_free_names): Ditto.
* fhandler_tty.cc (fhandler_tty_slave::fhandler_tty_slave): Don't set
unix_path_name here.
* path.cc (fchdir): Lock fd table throughout.  Use new
dtable::reset_unix_path_name method to reset path.
* syscalls.cc (stat_worker): Reorganize to always call fstat method.  Pass
path_conv method to fhandler_*::open.
(chroot): Elminate a goto.