Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

[ Tweaked subject prefix - Andrew ]
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

This patch does a number of things to help when failing to apply a new
configuration.

* The error body from the Unit control API is displayed which can have
  useful troubleshooting information (when the version of curl supports it).

* When using the EDIT option, the temporary file with unapplied changes is
  preserved so that the user can edit it again without losing their work.

* Editing JavaScript modules no longer requires that module to have been
  enabled.

* Failure to apply edited JavaScript modules now rolls-back to the previous
  configuration instead of deleting the module.

Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Now that unitd has multiple --control* startup options, locating the
address of the control socket requires additional precision.

Signed-off-by: Liam Crilly <liam.crilly@nginx.com>
Acked-by: Andrew Clayton <a.clayton@nginx.com>
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

Closes: <https://github.com/nginx/unit/pull/1062>

Previously, the edit method created a temporary file that was then sent
to curl(1) as --data-binary @filename.tmp. This did not work with
remote instances because the temporary file is not on the remote host.
The edit method now passes the configuration to curl(1) using stdin, the
same way as for all other configuration changes.

Introduces a new remote host scheme docker:// that specifies a local
container ID. By default, the control socket is assumed to be in the default
location, as per the Docker Official Images for Unit. If not, the path to
the control socket can be appended to the container ID.

Added --format option to manage configuration in other formats.
Initially, YAML is the only supported conversion format.
JSON/YAML conversion is performed with yq(1).

Suggested by: Torstein Krause Johansen <https://github.com/skybert>
Closes: #958 <https://github.com/nginx/unit/issues/958>

head -c 0 does not work on macOS (invalid byte count) but tail(1) is happy
to accept zero bytes, and does not have a performance penalty.