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]

terminal.c

@@ -2188,3 +2188,14 @@ void term_deselect (void) {
     deselect();
     term_update();
 }
+
+/*
+ * from_backend(), to get data from the backend for the terminal.
+ */
+void from_backend(int is_stderr, char *data, int len) {
+    while (len--) {
+	if (inbuf_head >= INBUF_SIZE)
+	    term_out();
+	inbuf[inbuf_head++] = *data++;
+    }
+}