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/file-move.sh

@@ -6,7 +6,7 @@ mkdir -p /tmp/mgmt/
 rm /tmp/mgmt/f1 &>/dev/null || true
 
 # run empty graph, with prometheus support
-$TIMEOUT "$MGMT" run --tmp-prefix yaml --yaml=file-move.yaml 2>&1 | tee /tmp/mgmt/file-move.log &
+$TIMEOUT "$MGMT" run --tmp-prefix yaml file-move.yaml 2>&1 | tee /tmp/mgmt/file-move.log &
 pid=$!
 sleep 5s	# let it converge