The most visible change is that the scrolling buffer of messages now
actually scrolls and keeps history, and you type on the bottom line only.
For now you can page through the history with ^P/^N; that's not ideal but
one can't fix everything all at once.
Use resizeable array code (roughly the same that I floated a while ago
in connection with make) to hold the scroll.
a.) Don't include a zillion system headers from "extern.h" (actually, don't
include any).
b.) Break a global jmp_buf variable into a new file, so <setjmp.h> doesn't
have to be pulled in everywhere (it's only used in two places).
The (one line) new file is (c) TNF with a three-clause license, FW(L)IW.
