mgpg: use quiet to only print errors

Else it can happen that gpg output is prepended to the message we
want to decode, and recursive decoding fails.

contrib/mgpg

@@ -20,7 +20,7 @@ n=$(mshow -t "$tmp" | awk -F: '
 	/: application\/octet-stream/ {if (supported) print $1}')
 
 if [ "$n" ]; then
-	mshow -O "$tmp" "$n" | $GPG -d 2>&1 || exit 0
+	mshow -O "$tmp" "$n" | $GPG --quiet -d 2>&1 || exit 0
 	exit 64
 fi
 exit 63