forward signals to children

[nlf]

this change adds handlers to ensure that signals are forwarded to child processes appropriately. handlers are not added until a process exists that should be monitored, and are removed as soon as all children have exited, carrying the first non-falsey exit code received from a child.

for: npm/cli#1947

[isaacs]

Some style nits, mostly. Only substantive bit is a question about how we handle multiple signals, and child process exiting (or not exiting) as a result of those signals. (The npm v6 way seems like the worst of both worlds.)