If the watchdog is sent SIGTERM from an external process then it
should stop accepting new connections and attempt to finish the
work in progress. This change makes use of the new ability in Go
1.9 and onwards to cancel a HTTP server gracefully.

The write_timeout duration is used as a grace period to allow all
in-flight requests to complete. The pattern is taken directly from
the offical example in the Golang documentation. [1]

Further tuning and testing may be needed for Windows containers which
have a different set of signals for closing work. This change aims
to cover the majority use-case for Linux containers.

The HTTP health-check is also invalidated by creating an and
expression with the existing lock file.

Tested with Kubernetes by deploying a custom watchdog and the
fprocess of `env`. Log message was observed when scaling down and
connections stopped being accepted on terminating replica.

Also corrects some typos from previous PR.

[1] https://golang.org/pkg/net/http/#Server.Shutdown



Signed-off-by: Alex Ellis (VMware) <alexellis2@gmail.com>