crypto/cipher: don't persist errors in StreamWriter.

I messed this up from the beginning. The receiver isn't a pointer so
setting Err is useless. In order to maintain the API, just remove the
superfluous code.

Fixes issue 4657.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7161043
crypto/x509: return a better error when we fail to load system roots.

R=golang-dev, krautz, rsc
CC=golang-dev
https://codereview.appspot.com/7157044
text/template/parse: don't show itemType in error messages
so that the user don't need to decipher something like this:
template: main:1: expected %!s(parse.itemType=14) in end; got "|"
now they get this:
template: main:1: unexpected "|" in end

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/7128054
cmd/5l: move offset2 into Adr.u0 union to save 4/8 bytes for Adr/Prog resp.
sizeof(Adr) from 24 bytes down to 20 bytes.
sizeof(Prog) from 84 bytes down to 76 bytes.

5l linking cmd/godoc statistics:
Before:
Maximum resident set size (kbytes): 106668
After:
Maximum resident set size (kbytes):  99412

R=golang-dev, dave
CC=golang-dev
https://codereview.appspot.com/7100059
cmd/cgo: doc updates
1. note that to use C.free <stdlib.h> must be included
2. can also extract errno from a void C function

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/6935045
doc/go_spec: cap doesn't apply to maps
Fixes issue 4682.

R=golang-dev, adg, dave
CC=golang-dev
https://codereview.appspot.com/7094062
runtime: account stop-the-world time in the "other" GOGCTRACE section
Currently it's summed to mark phase.
The change makes it easier to diagnose long stop-the-world phases.

R=golang-dev, bradfitz, dave
CC=golang-dev
https://codereview.appspot.com/7182043