runtime: fix running under nohup

There are two ways nohup(1) might be implemented:
it might mask away the signal, or it might set the handler
to SIG_IGN, both of which are inherited across fork+exec.
So two fixes:

* Make sure to preserve the inherited signal mask at
minit instead of clearing it.

* If the SIGHUP handler is SIG_IGN, leave it that way.

Fixes issue 4491.

R=golang-dev, mikioh.mikioh, iant
CC=golang-dev
https://codereview.appspot.com/7308102