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

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go