Allow retrieving some more kinds of clipboard data on Haiku

Allow retrieving some more kinds of clipboard data on Haiku

* src/haiku_select.cc (BClipboard_find_data): Handle NULL
characters inside data correctly.
(be_lock_clipboard_message, be_unlock_clipboard): New functions.
* src/haikuselect.c (Fhaiku_selection_data): Return entire
clipboard contents as a message when clipboard is NULL.
(haiku_lisp_to_message): Allow quitting when iterating through
potentially large or circular lists.
* src/haikuselect.h (enum haiku_clipboard): New enum.