log/syslog: retry once if write fails

Implements deferred connections + single-attempt automatic
retry. Based on CL 5078042 from kuroneko.

Fixes issue 2264.

R=mikioh.mikioh, rsc, bradfitz
CC=golang-dev
https://codereview.appspot.com/6782140

Committer: Brad Fitzpatrick <bradfitz@golang.org>