Implement wallpaper.el support for Haiku

Implement wallpaper.el support for Haiku

* lisp/image/wallpaper.el (haiku-set-wallpaper, wallpaper-set):
Use `haiku-set-wallpaper' on Haiku.
* lisp/term/haiku-win.el (haiku-write-node-attribute)
(haiku-send-message, haiku-set-wallpaper): New function.
* src/haiku_support.cc (be_write_node_message, be_send_message):
New functions.
* src/haiku_support.h: Update prototypes.
* src/haikuselect.c (haiku_message_to_lisp)
(haiku_lisp_to_message): Fix CSTR type handling to include NULL
byte.
(haiku_report_system_error, Fhaiku_write_node_attribute)
(Fhaiku_send_message): New functions.
(syms_of_haikuselect): Add defsubrs.