Fix freezes when trying to accelerate menu bar on Haiku

Fix freezes when trying to accelerate menu bar on Haiku

* src/haiku_support.cc (class EmacsWindow): New field
`menus_begun'.
(MenusBeginning): Don't send menu bar open events when that is
set, instead set it to true.
(BMenuBar_start_tracking): Stop locking the menu bar here and
send a special BE_MENU_BAR_OPEN event instead.
* src/haiku_support.h (struct haiku_menu_bar_state_event):
Delete field `no_lock'.
* src/haikumenu.c (Fhaiku_menu_bar_open):
* src/haikuterm.c (haiku_read_socket): Update accordingly.