Clean up Haiku file panel code

Clean up Haiku file panel code

* lisp/term/haiku-win.el (x-file-dialog): Fix nil values of
`default-filename'.
* src/haiku_io.c (haiku_len): Remove `FILE_PANEL_EVENT'.
(record_c_unwind_protect_from_cxx, c_specpdl_idx_from_cxx)
(c_unbind_to_nil_from_cxx): Delete functions.
* src/haiku_support.cc (MessageReceived): Write pointer to
buffer to file panel port instead.
(struct popup_file_dialog_data): Delete strict.
(unwind_popup_file_dialog): Delete functions.
(be_popup_file_dialog): Accept a pointer to
`process_pending_signals' and run nested event loop as usual.

* src/haiku_support.h (enum haiku_event_type): Remove
`FILE_PANEL_EVENT'.
(struct haiku_file_panel_event): Delete struct.
* src/haikufns.c (unwind_popup): Delete function.
(Fhaiku_read_file_name): Update and quit on invalid filename.

* src/haikuterm.c (struct unhandled_event): Delete struct.
(haiku_read_socket): Remove "unhandled events".