* path.cc (path_conv::check): Don't complain if /dev/x/foo when x doesn't

* path.cc (path_conv::check): Don't complain if /dev/x/foo when x doesn't
exist.
(mount_info::conv_to_win32_path): Keep translating when a /cygdrive is found.
Don't attempt to translate to a device name when devn == FH_CYGDRIVE.
(cygwin_conv_to_win32_path): Set buffer to empty on error.
(cygwin_conv_to_full_win32_path): Ditto.
* window.cc: Include unistd.h to verify definitions.