bufio: make Reader.Peek invalidate Unreads

Since Reader.Peek potentially reads from the underlying io.Reader,
discarding previous buffers, UnreadRune and UnreadByte cannot
necessarily work.  Change Peek to invalidate the unread buffers in all
cases (as allowed according to the documentation) and thus prevent
hiding bugs in the caller.

(This change was previoiusly merged and then reverted due concern about
being too close to a release)

Fixes #18556

Change-Id: I9027d75aa834d4b27703f37711ba25de04d89f3c
GitHub-Last-Rev: 917ef1e51131d734f92efc946a0ab5ca4ff69be6
GitHub-Pull-Request: golang/go#28768
Reviewed-on: https://go-review.googlesource.com/c/149297


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>