Community Roll-up

This is a community roll-up of advances others have contributed to their public forks of puma-dev.

Thanks to:

• @krasnoukhov for SIGTERM to shutdown apps (grabbed from @enova's fork)
• @nonrational for clean shutdown on timout
• @mlarraz for updating vendored fsnotify/fsevents library (grabbed from @enova's fork)
• @krasnoukhov for better handling of websocket headers
• @amasses for last line of log/stdout into error returned to browser on unexpected exit
• @while1malloc0 for saner env flags handling, abstracted defaults