command: prevent panic on graceful shutdown

[chrisroberts]

Description

When performing a graceful shutdown a channel is used to wait for
the agent to leave. The channel is closed when the agent leaves
successfully, but it also is closed within a deferral. If the
agent successfully leaves and closes the channel, a panic will
occur when the channel is closed the second time within the
deferral. To prevent this from occurring, the channel closing
is wrapped within a OnceFunc so the channel is only closed
once.

Testing & Reproduction steps

Panic encountered during shutdown:

panic: close of closed channel
goroutine 1 [running]:
github.com/hashicorp/nomad/command/agent.(*Command).terminateGracefully(0xc000948720, 0xc000538e70, {0x0?, 0x0?})
        github.com/hashicorp/nomad/command/agent/command.go:1019 +0x2a5
github.com/hashicorp/nomad/command/agent.(*Command).handleSignals(0xc000948720)
        github.com/hashicorp/nomad/command/agent/command.go:1069 +0x811
github.com/hashicorp/nomad/command/agent.(*Command).Run(0xc000948720, {0xc000072120, 0x2, 0x2})
        github.com/hashicorp/nomad/command/agent/command.go:909 +0xf25
github.com/hashicorp/cli.(*CLI).Run(0xc000a7cb40)
        github.com/hashicorp/[email protected]/cli.go:265 +0x4de
main.Run({0xc000072110, 0x3, 0x3})
        github.com/hashicorp/nomad/main.go:111 +0x225
main.main()
        github.com/hashicorp/nomad/main.go:82 +0x45

Contributor Checklist

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  and job configuration, please update the Nomad website documentation to reflect this. Refer to
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  change requires notes within the upgrade guide.

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[tgross]

LGTM!

[tgross]

LGTM!