time: avoid data race in locabs

time: avoid data race in locabs

This fixes a race condition discovered by:
        go build -race github.com/unixdj/benchnet/benchnode

The problem is essentially the same as issue 3967 (time:
data race on local Location), the fix being the same as
https://codereview.appspot.com/6460115/

Race trace:

==================
WARNING: DATA RACE
Write by goroutine 0:
  time.initLocal()
      src/pkg/time/zoneinfo_unix.go:49 +0x128
  sync.(*Once).Do()
      src/pkg/sync/once.go:40 +0x8d
  time.(*Location).get()
      src/pkg/time/zoneinfo.go:69 +0x70
  time.(*Location).lookup()
      src/pkg/time/zoneinfo.go:102 +0x4c
  time.Time.locabs()
      src/pkg/time/time.go:274 +0x179
  time.Time.Format()
      src/pkg/time/format.go:379 +0x60
  log/syslog.netConn.writeString()
      src/pkg/log/syslog/syslog.go:211 +0xc5
  log/syslog.(*netConn).writeString()
      src/pkg/log/syslog/syslog.go:0 +0xf9
  log/syslog.(*Writer).writeString()
      src/pkg/log/syslog/syslog.go:201 +0xd6
  log/syslog.(*Writer).Notice()
      src/pkg/log/syslog/syslog.go:181 +0x61
  main.main()
      github.com/unixdj/benchnet/benchnode/main.go:169 +0xa5e
  runtime.main()
      src/pkg/runtime/proc.c:248 +0x91

Previous read by goroutine 5:
  time.Time.locabs()
      src/pkg/time/time.go:270 +0x9a
  time.Time.Format()
      src/pkg/time/format.go:379 +0x60
  log/syslog.netConn.writeString()
      src/pkg/log/syslog/syslog.go:211 +0xc5
  log/syslog.(*netConn).writeString()
      src/pkg/log/syslog/syslog.go:0 +0xf9
  log/syslog.(*Writer).writeString()
      src/pkg/log/syslog/syslog.go:201 +0xd6
  log/syslog.(*Writer).Notice()
      src/pkg/log/syslog/syslog.go:181 +0x61
  main.talk()
      github.com/unixdj/benchnet/benchnode/proto.go:107 +0xc7
  main.netLoop()
      github.com/unixdj/benchnet/benchnode/main.go:103 +0x88

Goroutine 5 (running) created at:
  main.main()
      github.com/unixdj/benchnet/benchnode/main.go:158 +0xa04
  runtime.main()
      src/pkg/runtime/proc.c:248 +0x91