runtime: do not handle signals before configuring handler

runtime: do not handle signals before configuring handler

There was a small window during program initialization
where a signal could come in before the handling mechanisms
were set up to handle it.  Delay the signal-handler installation
until we're ready for the signals.

Fixes #3314.

R=golang-dev, dsymonds, mikioh.mikioh
CC=golang-dev
https://golang.org/cl/5833049