log/syslog: try /var/run/log, for FreeBSD

Fixes issue 8449.

LGTM=iant
R=golang-codereviews, iant
CC=bradfitz, golang-codereviews
https://codereview.appspot.com/145600044
compress/zlib: mention that NewReaderDict can return ErrDictionary

Fixes issue 7935.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews, ruiu
https://codereview.appspot.com/147390043
compress/*: note that NewReader may introduce buffering

Fixes issue 8309.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews, iant
https://codereview.appspot.com/147380043