runtime: fix build on linux

In addition to the compile failure fixed in signal*.c,
preserving the signal mask led to very strange crashes.
Testing shows that looking for SIG_IGN is all that
matters to get along with nohup, so reintroduce
sigset_zero instead of trying to preserve the signal mask.

TBR=iant
CC=golang-dev
https://codereview.appspot.com/7323067