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'.
