cmd/yacc: fix parsing of character tokens

From issue 7967 I learned:

1) yacc accepts either 'x' or "x" to mean token value 0x78
2) yacc also accepts 'xyz' and "XYZ" to mean token value 0x78

Use strconv.Unquote to simplify the handling of quoted
strings and check that each has only one rune.

Although this does clean things up, it makes 'x' and "x"
treated as different internally (now they are stored as
`'x'` and `"x"`; before they were both ` x`). Grammars that
use both interchangeably will now die with an error
similar to the one from issue 7967:

        yacc bug -- cannot have 2 different Ts with same value
                "+" and '+'

The echoing of the quotes should make clear what is going on.

The other semantic change caused by using strconv.Unquote
is that '\"' and "\'" are no longer valid. Like in Go, they must be
spelled without the backslash: '"' and "'".

On the other hand, now yacc and Go agree about what character
and string literals mean.

LGTM=r
R=r
CC=golang-codereviews
https://codereview.appspot.com/149110043

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