Unfix exitcode semantics.

src/runtime.cc

@@ -14,7 +14,7 @@ fn main (int argc, char **argv) -> int {
     int exitcode = 0;
     program_args = { (Cstr *) argv, (usize) argc };
     try {
-        exitcode = rustpp__main ().return_value ();
+        exitcode = rustpp__main ();
     } catch (panic::Panic p) {
         std::println ("Thread exited with explicit panic: {}", p.what ());
         exitcode = -1;