Fix graceful shutdown of containers fixes #947

[tnichols217]

Description of the Pull Request (PR):

Handles the SIGTERM signal to the starter binary such that containers have a chance to gracefully shut down, especially when run through a system service. Should fix issue #947, ableit a slightly different approach than what was described in the issue. This also allows us to properly close and stop mounts before we get SIGKILLed by the service if relevant.

This fixes or addresses the following GitHub issues:

• Fixes singularity not cleanup crypt when signal terminated #947

Before submitting a PR, make sure you have done the following:

• Read the Guidelines for Contributing, and this PR conforms to the stated requirements.
• Added changes to the CHANGELOG if necessary according to the Contribution Guidelines
• Added tests to validate this PR, linted with make check and tested this PR locally with a make test, and make testall if possible (see CONTRIBUTING.md).
• Based this PR against the appropriate branch according to the Contribution Guidelines
• Added myself as a contributor to the Contributors File

Done all of the above except writing tests, may need help with this as this is my first time contributing Go. This PR should also be squashed before merging.

[dtrudg]

Thanks for the attempted fix. Haven't had a chance to look into it in depth, but from the test failures, something isn't quite right.

The e2e tests are now timing out...

https://circleci.com/api/v1.1/project/github/sylabs/singularity/95681/output/120/0?file=true&allocation-id=6934a4e7b179c815fcd70cf7-0-build%2FABCDEFGH

Given that there's a RunSingularity call blocked for 29 minutes, it looks like the change is causing a deadlock for a / some tests...

goroutine 297 [chan receive, 29 minutes]:
testing.(*T).Run(0xc00080e000, {0xc000586ae0, 0x14}, 0xc0004a1bf0)
	testing/testing.go:2005 +0x9fe
github.com/sylabs/singularity/v4/e2e/internal/e2e.TestEnv.RunSingularity({{0xc00043b100, 0x1a}, {0xc000a40540, 0x1e}, {0xc0008b7290, 0x2b}, {0xc0008b72c0, 0x2f}, {0x0, 0x0}, ...}, ...)
	github.com/sylabs/singularity/v4/e2e/internal/e2e/singularitycmd.go:686 +0x45d
github.com/sylabs/singularity/v4/e2e/inspect.ctx.singularityInspect({{{0xc00043b100, 0x1a}, {0xc000a40540, 0x1e}, {0xc0008b7290, 0x2b}, {0xc0008b72c0, 0x2f}, {0x0, 0x0}, ...}}, ...)
	github.com/sylabs/singularity/v4/e2e/inspect/inspect.go:353 +0x2a50
github.com/sylabs/singularity/v4/e2e/internal/testhelper.(*Suite).Run.func1.1.1(0xc00080e000)
	github.com/sylabs/singularity/v4/e2e/internal/testhelper/testhelper.go:91 +0x3f
testing.tRunner(0xc00080e000, 0xc00005cc10)
	testing/testing.go:1934 +0x21d
created by testing.(*T).Run in goroutine 339
	testing/testing.go:1997 +0x9d3