builtin: document true and false.

Fixes issue 5315.

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8841048
doc: fix another broken link

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8763045
io: explain what (0,nil) means from Read
Also add a new variable ErrNoProgress that io.Readers can use to
report ineffectual Read calls.
Fixes issue 5310.

R=golang-dev, dsymonds, bradfitz
CC=golang-dev
https://codereview.appspot.com/8845043
doc/go1.1.html: document io.ErrNoProgress

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/8833050
bufio.Scan: don't stop after Read returns 0, nil
But stop eventually if the reader misbehaves.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/8757045
cmd/ld: don't show cursym in gcc invocation failure message
before:
fmt.statictmp_0731:/usr/local/go/pkg/tool/linux_amd64/6l: running gcc failed: unsuccessful exit status 0x100
now:
/usr/local/go/pkg/tool/linux_amd64/6l: running gcc failed: unsuccessful exit status 0x100

R=golang-dev, iant, r
CC=golang-dev
https://codereview.appspot.com/8827045
doc: fix another broken link

Fixes issue 5316

R=golang-dev, r, minux.ma
CC=golang-dev
https://codereview.appspot.com/8871043