Cygwin: Make native clipboard layout same for 32- and 64-bit

Cygwin: Make native clipboard layout same for 32- and 64-bit

This patch unifies the layout of the clipboard descriptor cygcb_t for
32- and 64-bit Cygwin.  It allows correct copy/paste between the two
environments without corruption of user's copied data and without access
violations due to interpreting that data as a size field.

The definitions of CYGWIN_NATIVE and cygcb_t are moved to a new include
file, sys/clipboard.h.  The include file is used by fhandler_clipboard.cc
as well as getclip.c and putclip.c in the Cygwin cygutils package.

When copy/pasting between 32- and 64-bit Cygwin environments, both must
be running version 3.3.0 or later for successful operation.