fix: handle SIGTERM gracefully on unix

[halaney]

We weren't catching SIGTERM and cancelling our context, so the go runtime would have us exit with a non-zero code when the signal was received. Let's catch it and try to exit gracefully.

i.e. before:

(...)
I0508 11:08:00.426917 1787442 log_monitor.go:237] Initialize condition generated: []
^C
$ echo $?
130

and now:

(...)
I0508 11:07:42.150562 1787146 log_monitor.go:237] Initialize condition generated: []
^C
I0508 11:07:43.372093 1787146 log_monitor.go:123] Stop log monitor /tmp/npd-test.json
I0508 11:07:43.652876 1787146 log_watcher.go:104] Stop watching filelog
I0508 11:07:43.652923 1787146 log_monitor.go:144] Log monitor stopped: /tmp/npd-test.json
$ echo $?
0

[k8s-ci-robot]

Hi @halaney. Thanks for your PR.

I'm waiting for a kubernetes member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: halaney
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