cmd/go: fix error for 'go install x.go' when GOBIN is not set

Fixes issue 6191.
Fixes issue 5426.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13234052
go/doc: restore handling of multi-paragraph BUG comments

It was lost when the generic "Notes" support went in.

Had to change the test setup, because it precluded even
being able test multi-line comments, much less multi-paragraph
comments.

Now 'godoc sync/atomic' works correctly again.

Fixes issue 6135.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13427045
cmd/go: run benchmarks in sequence

Fixes issue 5662.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13650043
cmd/go: fix go test using package main_test

A package main binary (that is, a command) being installed
does not mean we can skip the build of the package archive
during a test.

Fixes issue 3417.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13462046
cmd/go: show FAIL for errors during test setup

For example, if an x_test.go file contains a syntax error,
b.test fails with an error message. But it wasn't printing
the same FAIL line that a build failure later would print.
This makes all the test failures that happen (once we
decide to start running tests) consistently say FAIL.

Fixes issue 4701.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13431044
A+C: Arnaud Ysmal (individual CLA)

Generated by addca.

R=gobot
CC=golang-dev
https://codereview.appspot.com/13265044