Fix scroll bar portion on Haiku scroll bars

Fix scroll bar portion on Haiku scroll bars

* src/haiku_support.cc (EmacsScrollBar): Set steps to
appropriate value.
(ValueChanged): Test new value against old value before sending
value event.
(MessageReceived): Handle portion and range.
(BView_scroll_bar_update): New argument for portion.
* src/haiku_support.h: Update prototypes.
* src/haikuterm.c (haiku_set_scroll_bar_thumb):
(haiku_set_horizontal_scroll_bar_thumb): New functions.
(haiku_set_horizontal_scroll_bar):
(haiku_set_vertical_scroll_bar): Use those functions to set
scroll bar values.
(haiku_read_socket): Handle new meanings of scroll bar values.
* src/haikuterm.h (struct scroll_bar):