* fhandler.h (fhandler_dev_clipboard): Extend to support writing.

* fhandler.h (fhandler_dev_clipboard): Extend to support writing.
* fhandler_clipboard.cc (fhandler_dev_clipboard::fhandler_dev_clipboard):
Initialize new fields.  Open clipboard here.
(fhandler_dev_clipboard::dup): New method.
(fhandler_dev_clipboard::open): Accomodate new fields.  Register clipboard
here, if appropriate.
(set_clipboard): New function.  Moves buffer to clipboard.
(fhandler_dev_clipboard::write): Truly implement clipboard writing.
(fhandler_dev_clipboard::read): Reimplement to allow successive reads.
(fhandler_dev_clipboard::lseek): Truly implement seeks in clipboard.
(fhandler_dev_clipboard::close): Clear out new fields.  Support sequential
reads and sequential writes.  Support for binary data via a native clipboard
format.