fix signal handling

[mbostock]

Fixes #1459. The main things here are:

• Don’t try to record another signal if we’re already exiting
• Use process.kill instead of process.exit to ensure equivalence with default handler
• (Which requires using process.off to remove our handler before killing)

I also used Promise.race to simplify the code slightly.

[visnup]

Tested locally in terminal with INT, HUP, and TERM and matches previous behavior. Thanks!