The websocket interface used for `alloc exec` has to silently drop client send
errors because otherwise those errors would interleave with the streamed
output. But we may be able to surface errors that cause terminated websockets
a little better in the error returned and the HTTP server logs.

Handle when MemoryMaxMB is nil, as expected when a new 1.1.0 is hitting
a pre-1.1.0 Server.

docs: fixes broken links

Fixed broken links for the remote task driver ECS page

Move the words being defined in the /docs/internal/architecture page to be
small headers so that they can be linked to with anchors from Learn guides and
other documentation location.

merge from release

docs: run changelogfmt

docs: update enterprise license page

Added a link to the enterprise license tutorial and updated the trial link to use the recommended marketing url.

The `dist/` directory is unmaintained and gets out of sync with the Linux
packages that we distribute and the documentation found in the Learn
Guides. We even have upstart files for a distro that is out of support!
Remove this directory so that users are not confused.

While working on voxpupuli/puppet-nomad/pull/50 we observed that the released version of this file and the file in this repo are not in sync. This PR brings the systemd service file in sync with what is currently packaged into the rpm and deb packages as released in the HashiCorp repositories. I am not sure how/where to update the  [deployment guide](https://learn.hashicorp.com/tutorials/nomad/production-deployment-guide-vm-with-consul#configure-systemd)

drivers/exec+java: reduce default set of linux capabilities

drivers/exec+java: reduce default set of linux capabilities

Add capabilities to the LaunchRequest proto so that the
capabilities set actually gets plumbed all the way through
to task launch.

Looks like we no longer need a package.

The error output being checked depends on the linux caps supported
by the particular operating system. Fix these test cases to just
check that an error did occur.

Update docs for allow_caps, cap_add, cap_drop in exec/java/docker driver
pages. Also update upgrade guide with guidance on new default linux
capabilities for exec and java drivers.

This changeset does not introduce any functional change for the
docker driver, but rather cleans up the implementation around
computing configured capabilities by re-using code written for
the exec/java task drivers.

Enable setting allow_caps on the java task driver plugin, along
with the associated cap_add and cap_drop options in java task
configuration.

This PR enables setting allow_caps on the exec driver
plugin configuration, as well as cap_add and cap_drop in
exec task configuration. These options replicate the
functionality already present in the docker task driver.

Important: this change also reduces the default set of
capabilities enabled by the exec driver to match the
default set enabled by the docker driver. Until v1.0.5
the exec task driver would enable all capabilities supported
by the operating system. v1.0.5 removed NET_RAW from that
list of default capabilities, but left may others which
could potentially also be leveraged by compromised tasks.

Important: the "root" user is still special cased when
used with the exec driver. Older versions of Nomad enabled
enabled all capabilities supported by the operating system
for tasks set with the root user. To maintain compatibility
with existing clusters we continue supporting this "feature",
however we maintain support for the legacy set of capabilities
rather than enabling all capabilities now supported on modern
operating systems.

Remote Task Driver docs