Add flag to register shutdown hooks for os.call and os.spawn APIs, overhaul destroy APIs

[lihaoyi]

• Backports the functionality from Mill to allow us to register shutdown hooks such that when the parent process terminates the subprocesses are shut down as well. This allows the same logic to be used consistently across all .call and .spawn invocations

• Consolidate SubProcess#destroy and SubProcess#destroyForcibly into a single SubProcess#destroy method which takes some default parameters, allowing the user to choose async = true or configure the shutdownGracePeriod: Long.

• The default behavior of SubProcess#destroy has changed to block on the subprocess actually exiting, which I think is more intuitive. The old behavior is available under destroy(async = true)

Best reviewed with Hide whitespace. Added some simple unit tests to assert the new functionality, existing unit tests should assert on existing workflows

[lefou]

I think we should rename shutdownHook to something like exitWithJvm or stopWithJvm. We don't really support any custom shtudown hook. We just use the shutdown hook mecahnism internally to stop the process when the JVM is stopped.

[lihaoyi]

Ended up going with destroyOnExit, to match the existing os.temp(deleteOnExit = true) and .destroy() methods

[lefou]

What's the time unit used for shutdownGracePeriod? Should we name it shutdownGracePeriodMsec?

[lihaoyi]

I'll probably leave it as is, the scaladoc says it's in milliseconds so that should be enough

[lefou]

Is this only provided for bin-compat? Shouldn't we deprecate it therefore?

[lihaoyi]

I've had issues with @deprecated things being accidentally resolved here before, but will add a comment like the other forwarders

[lefou]

Looks like this one is only for bin-compat, so we should deprecate it for removal.