runtime: start all threads with runtime.mstart

Putting the M initialization in multiple places will not scale.
Various code assumes mstart is the start already. Make it so.

R=golang-dev, devon.odell
CC=golang-dev
https://codereview.appspot.com/7420048
syscall: disable TestPassFD on openbsd

Update issue 4956.

R=golang-dev
CC=golang-dev
https://codereview.appspot.com/7417048