Clean up child process handling

[slackpad]

This issue consolidates several similar issues with regards to child process handling. There are several areas in Consul where we spawn child processes but we have various issues:

• consul exec
• consul lock
• consul watch
• watch scripts
• health check scripts

There are cases where we don't properly clean up and can leak child processes, or we don't handle signals properly. Some places we assume a shell, and others we don't. We should factor child process handling to be done in a uniform way (look at https://github.com/hashicorp/consul-template/tree/master/child, and maybe use that) and make sure that:

• Signals are handled properly and passed along to child processes
• Users can run without a shell if desired (we can have an optional shell to keep backward compatibility) (from Ability to configure shells for health check scripts #1358)
• Orphaned processes aren't left around
• Quoting works on Windows (consul exec should not escape quotes on windows platform #1923)
• stdin works on Windows (stdin for consul exec doesn't work on windows #1924)

[maxenglander]

@slackpad not sure if the following issue is covered by this master ticket, but something I've observed is that consul watch doesn't return the exit code of it's handler. E.g.

$ consul watch -prefix foo/bar sh -c 'exit 2'; echo $?
Error executing handler: exit status 2
1

This makes it difficult to integrate consul watch into a supervisor process that takes certain actions or not (e.g. restart or not) based on exit codes.

Let me know if it's not covered, I'll happily open a new ticket.

[slackpad]

Hi @maxenglander that's not captured here and is probably better on a separate issue. A similar change for consul lock was done under #947.

[maxenglander]

Aha! Thanks @slackpad, opened #3526.