Send the IUTF8 terminal mode in SSH "pty-req"s.

An opcode for this was recently published in
https://tools.ietf.org/html/draft-sgtatham-secsh-iutf8-00 .

The default setting is conditional on frontend_is_utf8(), which is
consistent with the pty back end's policy for setting the same flag
locally. Of course, users can override the setting either way in the
GUI configurer, the same as all other tty modes.

ssh.c

@@ -125,6 +125,7 @@ static const struct ssh_ttymode {
     { "IXANY",	     39, TTY_OP_BOOL },
     { "IXOFF",	     40, TTY_OP_BOOL },
     { "IMAXBEL",     41, TTY_OP_BOOL },
+    { "IUTF8",       42, TTY_OP_BOOL },
     { "ISIG",	     50, TTY_OP_BOOL },
     { "ICANON",	     51, TTY_OP_BOOL },
     { "XCASE",	     52, TTY_OP_BOOL },