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/prometheus-3.sh

@@ -5,7 +5,7 @@
 exit 0	# XXX: temporarily disabled until prometheus is added back post refactor
 
 # run a graph, with prometheus support
-$TIMEOUT "$MGMT" run --tmp-prefix --no-pgp --prometheus yaml --yaml prometheus-3.yaml &
+$TIMEOUT "$MGMT" run --tmp-prefix --no-pgp --prometheus yaml prometheus-3.yaml &
 pid=$!
 sleep 10s	# let it converge