When rendering a task consul template, ensure that only task environment
variables are used.

Currently, `consul-template` always falls back to host process
environment variables when key isn't a task env var[1].  Thus, we add
an empty entry for each host process env-var not found in task env-vars.

[1] https://github.com/hashicorp/consul-template/blob/bfa5d0e133688920afd1e012404f765182e3d5e0/template/funcs.go#L61-L75

Defend against tr.getDriverHandle being nil.  Exec handler checks if
task is running, but it may be stopped between check and driver handler
fetching.

Co-Authored-By: Mahmood Ali <mahmood@hashicorp.com>

Adds a new Prerun and Postrun hooks to manage set up of network namespaces
on linux. Work still needs to be done to make the code platform agnostic and
support Docker style network initalization.

comment formattinglogmon: fix comment formattinglogmon: fix comment
formatting

...and add a comment to switch back to the old error handling once we
switch to Go 1.12.

There's a bug in go1.11 that causes some io operations on windows to
return incorrect errors for some cases when Stat-ing files. To avoid
upgrading to go1.12 in a point release, here we loosen up the cases
where we will attempt to create fifos, and add some logging of
underlying stat errors to help with debugging.

To pick up the fix in
https://github.com/hashicorp/consul-template/pull/1231 .

Previously, if a channel is closed, we retry the Stats call.  But, if that call
fails, we go in a backoff loop without calling Stats ever again.

Here, we use a utility function for calling driverHandle.Stats call that retries
as one expects.

I aimed to preserve the logging formats but made small improvements as I saw fit.

This makes for a nicer task event message

Handle when prestart failed while restoring a task, to prevent
accidentally leaking consul/logmon processes.