feat(users/Profpatsch/execline/exec_helpers): add no_args()

Some programs don’t need any arguments, so fail if they do get them,
because that’s usually a bug.

Change-Id: I28639056d3d9cea0cc0e7fcbfa42120c4f129c8c
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2503
Tested-by: BuildkiteCI
Reviewed-by: Profpatsch <mail@profpatsch.de>

users/Profpatsch/read-http.rs

@@ -22,6 +22,8 @@ enum What {
 // The keys are text, but can be lists of text iff headers appear multiple times, so beware.
 fn main() -> std::io::Result<()> {
 
+    exec_helpers::no_args("read-http");
+
     let args = dec::RecordDot {
         field: "what",
         inner: dec::OneOf {