Make curl example into a complete tool (#706)

You can now handily use `curl` with popen()/system() by
placing `STATIC_YOINK("_curl")` in your main source file.

tool/curl/cmd.c

@@ -0,0 +1,5 @@
+#include "tool/curl/cmd.h"
+
+int main(int argc, char *argv[]) {
+  return _curl(argc, argv);
+}