Add shell completion with cobra

Allow automatic generation for shell completion scripts
with the internal cobra functions (requires v1.0.0+).

This should replace the handwritten completion scripts
and even adds support for fish. With this approach it is
less likley that completions and code are out of sync.

We can now create the scripts with
- podman completion bash
- podman completion zsh
- podman completion fish

To test the completion run:
source <(podman completion bash)

The same works for podman-remote and podman --remote and
it will complete your remote containers/images with
the correct endpoints values from --url/--connection.

The completion logic is written in go and provided by the
cobra library. The completion functions lives in
`cmd/podman/completion/completion.go`.

The unit test at cmd/podman/shell_completion_test.go checks
if each command and flag has an autocompletion function set.
This prevents that commands and flags have no shell completion set.

This commit does not replace the current autocompletion scripts.

Closes #6440

Signed-off-by: Paul Holzinger <paul.holzinger@web.de>

hack/podman-commands.sh

@@ -25,6 +25,12 @@ function podman_commands() {
     $PODMAN help "$@" |\
         awk '/^Available Commands:/{ok=1;next}/^Options:/{ok=0}ok { print $1 }' |\
         grep .
+
+    # Special case: podman-completion is a hidden command
+    # it does not show in podman help so add it here
+    if [[ -z "$@" ]]; then
+        echo "completion"
+    fi
 }
 
 # Read a list of subcommands from a command's metadoc

hack/xref-helpmsgs-manpages

@@ -181,6 +181,9 @@ sub xref_by_man {
             next if $k eq 'varlink';
             next if "@subcommand" eq 'system' && $k eq 'service';
 
+            # Special case: podman completion is a hidden command
+            next if $k eq 'completion';
+
             warn "$ME: podman @subcommand: $k in $man, but not --help\n";
             ++$Errs;
         }