use when they have data from the network. Replaces the utterly daft
inbuf / inbuf_head / term_out() interface, which only made sense
when feeding to terminal.c. (terminal.c now implements
from_backend() as a small function that gateways to the old
interface.)
As a side effect, from_backend() also has an `is_stderr' parameter,
so scp can once again separate the server's pronouncements on stderr
from the actual protocol progress on stdout.
[originally from svn r729]
windlg.c into it. Allows plink and pscp to no longer link with
windlg.c, meaning they lose some of the sillier stub functions and
also can provide a console-based form of verify_ssh_host_key().
[originally from svn r683]
Also we are able to notice when a backend is instantly sendok(),
rather than waiting until after the first successful socket read.
(This was zogging raw connections. They're still slightly zogged but
not as badly as they were.)
[originally from svn r671]
