Change SIGINT to SIGTERM

[cbednarski]

The docs casually mention that os.Interrupt doesn't do anything on Windows, so I'm curious whether proc.Signal does anything at all on Windows or is silently ignored.

unix.SIGTERM should work on any POSIX OS.

Closes #27

[ryanuber]

Can we not just use the constants in syscall here? syscall.SIGTERM seems like it could help us avoid the extra import.

[cbednarski]

The syscall docs say this:

NOTE: This package is locked down. Code outside the standard Go repository should be migrated to use the corresponding package in the golang.org/x/sys repository.

[mitchellh]

I'd probably use a build tag file signal_windows.go signal_linux.go etc. to set a variable that we use to set that signal. I don't see a need to depend on that other package for this. And on Windows its probably something else, and I imagine some point in 2016 we'll get this thing to work on Windows too (Windows Server Containers with Docker)

[cbednarski]

After digging further, the go signal implementation for windows doesn't handle signals at all. They all result in an error state. We don't check errors from the signal call so all this means for windows is we wait 5 seconds and then kill the process.

TL;DR this change makes POSIX systems use SIGTERM, and Windows is completely unaffected.

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