* termhooks.h (enum scroll_bar_part): Begin from 0 to allow...

* termhooks.h (enum scroll_bar_part): Begin from 0 to allow...
(struct input_event): ...unsigned bitfields.  Likewise for
`event_kind' member.  Prefer unsigned for `code' and 'modifiers'.
Use `timestamp' for HELP_EVENT position.  Add compile-time assert.
* keyboard.c (gen_help_event, kbd_buffer_store_help_event)
(kbd_buffer_get_event): Adjust users.
(scroll_bar_parts): Add Qnil to match scroll_bar_nowhere.
(make_scroll_bar_position): New function, refactored out of...
(make_lispy_event): ...adjusted user.
* nsterm.h (EmacsScroller): Use enum for `last_hit_part' member.
* nsterm.m (ns_mouse_position, mouseUp):
* term.c (term_mouse_position):
* w32inevt.c (w32_console_mouse_position):
* w32term.c (w32_mouse_position):
* xterm.c (XTmouse_position): Use scroll_bar_above_handle.
(x_send_scroll_bar_event, xm_scroll_callback, xg_scroll_callback):
Prefer enum and explicit enum members to integers and numeric values.