run: executed cmd is not terminated when baur process dies

[fho]

How to reproduce:

• run a long running process (sleep 10m) as baur task,
• send SIGTERM to the baur process

result:

• the sleep process is still running

expected result:

• the executed task terminates when the baur process terminates

Fixed:

• Linux & FreeBSD: run: ensure task process is killed when baur terminates #404
• windows: syscall: add support for Windows job objects golang/go#17608 support required?
• mac os: ?