* include/winioctl.h (FSCTL_SET_SPARSE): Define.
cygwin:
* wincap.h (wincaps::supports_sparse_files): New flag.
(wincapc::supports_sparse_files): New method.
* wincap.cc (wincap_unknown): Define value for the new flag.
(wincap_95): Ditto.
(wincap_95osr2): Ditto.
(wincap_98): Ditto.
(wincap_98se): Ditto.
(wincap_me): Ditto.
(wincap_nt3): Ditto.
(wincap_nt4): Ditto.
(wincap_nt4sp4): Ditto.
(wincap_2000): Ditto.
(wincap_xp): Ditto.
* path.h (path_conv::fs_flags): New method.
* fhandler_disk_file.cc: Include winioctl.h for DeviceIoControl.
(fhandler_disk_file::open): Set newly created and truncated files as
sparse on platforms that support it.