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/load0.sh

@@ -37,7 +37,7 @@ file "${tmpdir}/loadavg" {
 }
 EOF
 
-$TIMEOUT "$MGMT" run --tmp-prefix --converged-timeout=15 lang --lang "$tmpdir/load0.mcl"  &
+$TIMEOUT "$MGMT" run --tmp-prefix --converged-timeout=15 lang "$tmpdir/load0.mcl"  &
 pid=$!
 wait $pid	# get exit status
 e=$?