* shortcut.c: New file. Provides a C interface to reading of

    * shortcut.c: New file. Provides a C interface to reading of
        Windows shortcuts to avoid compiler flag `-fvtable-thunks'.
        * shortcut.h: Ditto.
        * Makefile.in: Add shortcut.o to DLL_OFILES.
        * cygerrno.h: Provide a C interface to `geterrno_from_win_error' for
        using in shortcut.c.
        * errno.cc (geterrno_from_win_error): Define as extern "C".
        * path.cc (struct symlink_info): Remove methods `check_shortcut' and
        `check_sysfile'.
        (shortcut_header): Move to shortcut.c.
        (shortcut_initalized): Ditto.
        (create_shortcut_header): Ditto.
        (cmp_shortcut_header): Ditto.
        (symlink_info::check_shortcut): Ditto. Reorganize as a plain C function.
        (symlink_info::check_sysfile): Redefine as a global function using the
        same parameter list as `check_shortcut' for clearness.
        (symlink_info::check): Change parameter list for calls to
        `check_shortcut' and `check_sysfile'.