rustpp_main: Fix exitcode semantics

src/runtime.cc

@@ -11,12 +11,15 @@ namespace {
 }
 
 fn main (int argc, char **argv) -> int {
+    int exitcode = 0;
     program_args = { (Cstr *) argv, (usize) argc };
     try {
-        return rustpp__main ();
+        exitcode = rustpp__main ().return_value ();
     } catch (panic::Panic p) {
         std::println ("Thread exited with explicit panic: {}", p.what ());
+        exitcode = -1;
     }
+    return exitcode;
 }
 
 // Environment