Improve js_os_exec (#295)

- use $(shell) make command to test if closefrom() is available
- use closefrom() if available in js_os_exec()
- limit the fallback loop to 1024 handles to avoid costly loop on linux alpine.
PR inspired by @nicolas-duteil-nova

compat/test-closefrom.c

@@ -0,0 +1,6 @@
+#include <unistd.h>
+
+int main(void) {
+    closefrom(3);
+    return 0;
+}