add QuoteToASCII.
The Quote and QuoteRune functions now let printable
runes (as defined by unicode.IsPrint) through.  When
true 7-bit clean stuff is necessary, there are now two
new functions: QuoteToASCII and QuoteRuneToASCII.

Printf("%q") uses Quote. To get the old behavior, it
will now be necessary to say
        Printf("%s", strconv.QuoteToASCII(s))
but that should rarely be necessary.

R=golang-dev, gri, r
CC=golang-dev
https://golang.org/cl/4561061