* cygerrno.h: New file. Use this throughout whenever errno manipulation is

* cygerrno.h: New file.  Use this throughout whenever errno manipulation is
required.
* errno.cc: Use DWORD to hold Windows errors.
(geterrno_from_win_error): New function.
(seterrno_from_win_error): Use geterrno_from_win_error to convert supplied
windows error (suggested by Corinna Vinschen).
* path.cc (symlink_info): Add error element.
* path.cc (path_conv::check): Remove errno setting.  Use new symlink_info errno
element to set path_conv error, where appropriate.
(symlink_info::check): Set error element rather than attempting to manipulate
errno.  Add more checks for trailing / and /..  even though they are currently
useless.  Avoid setting EINVAL.
* path.cc (normalize_posix_path): Correct check for trailing /.