The previous fix to `consul lock` (commit 6875e8d) didn't completely
eliminate the race that could occur if the lock was acquired around the
same time SIGTERM was received: It was still possible for
Run() to spawn the process via startChild() after killChild() had
released the shared mutex.
Now, when SIGTERM is received, we acquire a mutex that prevents
spawning a new process and never release it.
We've tested this fix pretty thoroughly and believe it completely
resolves the issue.
Add a per GOOS implementation to send signals to PIDs
since syscall.Kill does not exist on Windows.
It will always send a SIGKILL on Windows since there
seems to be no POSIX compatible notion.
Fixes build on Windows.