Introduce a sane interface function, from_backend(), for backends to

Introduce a sane interface function, from_backend(), for backends to
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]