all: Switch the --lang syntax to use argv instead

It was a bit awkward using `mgmt run lang --lang <input>` so instead, we
now drop the --lang, and read the positional argv for the input.

This also does the same for the --yaml frontend.

test/shell/yaml-change1.sh

@@ -16,7 +16,7 @@ fi
 
 # set the config file
 cp -a yaml-change1a.yaml /tmp/mgmt/yaml-change.yaml
-$TIMEOUT "$MGMT" run --tmp-prefix yaml --yaml /tmp/mgmt/yaml-change.yaml &
+$TIMEOUT "$MGMT" run --tmp-prefix yaml /tmp/mgmt/yaml-change.yaml &
 pid=$!
 sleep 5s	# let it converge
 grep -q 'hello world' /tmp/mgmt/change1	# check contents are correct