upstream fixes for accept-loop error handling

Restore some `exec` behavior 

WIP: fix failing tests caused by async driver manager

Restores pre-0.9 behavior, where Nomad makes /dev available to exec
task.  Switching to libcontainer, we accidentally made only a small
subset available.

Here, we err on the side of preserving behavior of 0.8, instead of going
for the sensible route, where only a reasonable subset of devices is
mounted by default and user can opt to request more.

libcontainer based drivers (e.g. exec, java) should default to running
processes as `nobody` [1]; but libcontainer treats empty user as `root`
in our case (either because of default or due to `root` being current
user).

[1] https://github.com/hashicorp/nomad/blob/94c28a4c6cc45f1b377d50a6bff2017b46c43d18/website/source/docs/job-specification/task.html.md#task-parameters

allocrunner: Standardised discard logs

taskrunner: Persist environment from hooks

Fix `make check`

Follow up from https://github.com/hashicorp/nomad/pull/5007#pullrequestreview-186739124

drivers: Cleanup root user fingerprinting

https://github.com/hashicorp/nomad/pull/5032 introduced a regression
where the origHookState was used in place of the response from the hook.

e2e tests for spread stanza

Store device envs separately and pass to drivers

client: add proper build flag to allocrunner testing.go

This is an unrecoverable error, so we should only do this check once,
rather than returning unhealthy constantly.

This is an unrecoverable error, so we should only do this check once,
rather than returning unhealthy constantly.

ci: use gotestsum for running tests