Define and use sbbs_t::msghdr_tos (exposed via JS as

console.msghdr_top_of_screen):
a hack to let scripts know whether the most-recently displayed message header
was displayed on the top of the terminal screen (or not). This allows us to
show avatars at the top of the screen when the header is up there, rather than
relevative to the bottom of the header (the pos can vary, depending on stuff).