runtime: lock the main goroutine to the main OS thread during init

runtime: lock the main goroutine to the main OS thread during init

We only guarantee that the main goroutine runs on the
main OS thread for initialization.  Programs that wish to
preserve that property for main.main can call runtime.LockOSThread.
This is what programs used to do before we unleashed
goroutines during init, so it is both a simple fix and keeps
existing programs working.

R=iant, r, dave, dvyukov
CC=golang-dev
https://golang.org/cl/5309070