cmd/go: check GOROOT directory is present before acting

Fixes issue 5042.

R=golang-dev, adg, rsc
CC=golang-dev
https://codereview.appspot.com/7786047
syscall: add IPv6 socket options for Unix variants

This CL adds missing IPv6 socket options which are required
to control IPv6 as described in RFC 3493, RFC 3542.

Update issue 5538

R=golang-dev, dave, iant
CC=golang-dev
https://codereview.appspot.com/9373046
crypto/rsa: implement PSS signatures.

This change contains an implementation of the RSASSA-PSS signature
algorithm described in RFC 3447.

R=agl, agl
CC=gobot, golang-dev, r
https://codereview.appspot.com/9438043

Committer: Adam Langley <agl@golang.org>
io: Prioritize WriterTos over ReaderFroms in Copy.

This only affects calls where both ReaderFrom and WriterTo are implemented. WriterTo can issue one large write, while ReaderFrom must Read until EOF, potentially reallocating when out of memory. With one large Write, the Writer only needs to allocate once.

This also helps in ioutil.Discard since we can avoid copying memory when the Reader implements WriterTo.

R=golang-dev, dsymonds, remyoudompheng, bradfitz
CC=golang-dev, minux.ma
https://codereview.appspot.com/9462044
misc/emacs: Do not modify kill ring when programmatically deleting text

Operations like gofmt and go-remove-unused-imports delete entire
lines of text. Previously this put them on the kill-ring,
negatively affecting user experience.

R=adonovan
CC=gobot, golang-dev
https://codereview.appspot.com/9605043

Committer: Brad Fitzpatrick <bradfitz@golang.org>
codereview: close the head tag

R=golang-dev, dsymonds
CC=golang-dev
https://codereview.appspot.com/9695045
A+C: Frederick Kelly Mayle III (individual CLA)

Generated by addca.

R=gobot
CC=golang-dev
https://codereview.appspot.com/9669044
runtime: faster range on empty map

benchmark                old ns/op    new ns/op    delta
BenchmarkMapIter               191          190   -0.52%
BenchmarkMapIterEmpty           22            4  -78.96%

R=golang-dev, minux.ma, dvyukov, iant, khr
CC=golang-dev
https://codereview.appspot.com/9637043

Committer: Brad Fitzpatrick <bradfitz@golang.org>
Merge branch 'master' into gitfix