Reland "Add optional signal forwarder and '-z' option to cli"

This is a reland of https://android-review.googlesource.com/#/c/365114/,
using NSIG instead of SIGUNUSED.

This reverts commit 7151582f0b847b62ec74c600dd6b99e8a0078c75.

Bug: None
Test: Build for ARM, MIPS, x86.
Change-Id: I5135c5c36fb4f98467790efaa0cba6d82e2ca2b6
3 files changed