Rework handling of untrusted terminal data.

Now there's a centralised routine in misc.c to do the sanitisation,
which copies data on to an outgoing bufchain. This allows me to remove
from_backend_untrusted() completely from the frontend API, simplifying
code in several places.

Two use cases for untrusted-terminal-data sanitisation were in the
terminal.c prompts handler, and in the collection of SSH-2 userauth
banners. Both of those were writing output to a bufchain anyway, so
it was very convenient to just replace a bufchain_add with
sanitise_term_data and then not have to worry about it again.

There was also a simplistic sanitiser in uxcons.c, which I've now
replaced with a call to the good one - and in wincons.c there was a
FIXME saying I ought to get round to that, which now I have!

pscp.c

@@ -196,15 +196,6 @@ int from_backend(Frontend *frontend, int is_stderr,
 
     return 0;
 }
-int from_backend_untrusted(Frontend *frontend, const void *data, int len)
-{
-    /*
-     * No "untrusted" output should get here (the way the code is
-     * currently, it's all diverted by FLAG_STDERR).
-     */
-    assert(!"Unexpected call to from_backend_untrusted()");
-    return 0; /* not reached */
-}
 int from_backend_eof(Frontend *frontend)
 {
     /*