Treat DEL(\x7f) as a control character

It seems that iTerm2 is able to invoke US-ASCII or DEC Special set into GL area.
Both of them are 94 character sets. So \x7f should be treated as a control character(DEL).

Now, the following code prints "abc de" on iTerm2.

> echo -e "abc\x7fde"

But it will be "abcde" on most terminals(such as xterm, Terminal.app).
This patch improves the compatibility among iTerm2 and them.