Allow dragging scroll bar for overscroll on Haiku

Allow dragging scroll bar for overscroll on Haiku

* src/haiku_support.cc (class EmacsScrollBar): New fields
`in_overscroll', `can_overscroll', `last_overscroll',
`last_reported_overscroll_value' and `max_value'.
(ValueChanged): Make very sure extraneous value are not
generated.
(MouseUp): Clear overscroll if enabled.
(MouseMoved): If overscroll is enabled and dragging downwards
fails to dislodge the maximum value, enable overscrolling.
(BView_scroll_bar_update): New parameter `can_overscroll'
* src/haiku_support.h: Update prototypes.
* src/haikuterm.c (haiku_set_scroll_bar_thumb)
(haiku_set_horizontal_scroll_bar_thumb): Adjust parameters to
scroll bar update functions.
(haiku_read_socket): Take ceiling of bar position if
horizontal.