fix popen, improve tests

src/header/stdio/mod.rs

@@ -917,9 +917,15 @@ pub unsafe extern "C" fn popen(command: *const c_char, mode: *const c_char) -> *
         //TODO: dup errors are ignored, should they be?
         {
             if write {
-                unistd::dup2(0, pipes[0]);
+                match unistd::dup2(pipes[0], 0) {
+                    0 => {}
+                    e => stdlib::exit(127),
+                }
             } else {
-                unistd::dup2(1, pipes[1]);
+                match unistd::dup2(pipes[1], 1) {
+                    1 => {}
+                    e => stdlib::exit(127),
+                }
             }
 
             unistd::close(pipes[0]);