Add log reading to the journald log driver

If a logdriver doesn't register a callback function to validate log
options, it won't be usable.  Fix the journald driver by adding a dummy
validator.

Teach the client and the daemon's "logs" logic that the server can also
supply "logs" data via the "journald" driver.  Update documentation and
tests that depend on error messages.

Add support for reading log data from the systemd journal to the
journald log driver.  The internal logic uses a goroutine to scan the
journal for matching entries after any specified cutoff time, formats
the messages from those entries as JSONLog messages, and stuffs the
results down a pipe whose reading end we hand back to the caller.

If we are missing any of the 'linux', 'cgo', or 'journald' build tags,
however, we don't implement a reader, so the 'logs' endpoint will still
return an error.

Make the necessary changes to the build setup to ensure that support for
reading container logs from the systemd journal is built.

Rename the Jmap member of the journald logdriver's struct to "vars" to
make it non-public, and to make it easier to tell that it's just there
to hold additional variable values that we want journald to record along
with log data that we're sending to it.

In the client, don't assume that we know which logdrivers the server
implements, and remove the check that looks at the server.  It's
redundant because the server already knows, and the check also makes
using older clients with newer servers (which may have new logdrivers in
them) unnecessarily hard.

When we try to "logs" and have to report that the container's logdriver
doesn't support reading, send the error message through the
might-be-a-multiplexer so that clients which are expecting multiplexed
data will be able to properly display the error, instead of tripping
over the data and printing a less helpful "Unrecognized input header"
error.

Signed-off-by: Nalin Dahyabhai <nalin@redhat.com> (github: nalind)

docs/reference/api/docker_remote_api_v1.18.md

@@ -279,7 +279,7 @@ Json Parameters:
         systems, such as SELinux.
     -   **LogConfig** - Log configuration for the container, specified as
           `{ "Type": "<driver_name>", "Config": {"key1": "val1"}}`.
-          Available types: `json-file`, `syslog`, `none`.
+          Available types: `json-file`, `syslog`, `journald`, `none`.
           `json-file` logging driver.
     -   **CgroupParent** - Path to cgroups under which the cgroup for the container will be created. If the path is not absolute, the path is considered to be relative to the cgroups path of the init process. Cgroups will be created if they do not already exist.
 
@@ -480,7 +480,7 @@ Status Codes:
 Get stdout and stderr logs from the container ``id``
 
 > **Note**:
-> This endpoint works only for containers with `json-file` logging driver.
+> This endpoint works only for containers with the `json-file` or `journald` logging drivers.
 
 **Example request**:
 

docs/reference/api/docker_remote_api_v1.19.md

@@ -493,7 +493,7 @@ Status Codes:
 Get `stdout` and `stderr` logs from the container ``id``
 
 > **Note**:
-> This endpoint works only for containers with `json-file` logging driver.
+> This endpoint works only for containers with the `json-file` or `journald` logging drivers.
 
 **Example request**:
 

docs/reference/api/docker_remote_api_v1.20.md

@@ -504,7 +504,7 @@ Status Codes:
 Get `stdout` and `stderr` logs from the container ``id``
 
 > **Note**:
-> This endpoint works only for containers with `json-file` logging driver.
+> This endpoint works only for containers with the `json-file` or `journald` logging drivers.
 
 **Example request**: