start-vnc: compatibility with MacOS

start-vnc

@@ -300,7 +300,7 @@ main () {
     local vnc_port_local=$((5900 + RANDOM % 100))
     local vnc_port_remote=$(( 5900 + vnc_display ))
     until ssh -o ForwardX11=no \
-              -o StrictHostKeyChecking=accept-new \
+              -o StrictHostKeyChecking=ask \
               -L "${vnc_port_local}:localhost:${vnc_port_remote}" \
               -Nf \
               "${SVNC_USERNAME}@${vnc_host}.dkrz.de"; do