Introduce addprocs_scyld and support TCP and UDP in init_stdio_handle()

Leveraging a more natural I/O pattern with scyld required TCP and UDP support in init_stdio_handle() (which was trivial enough to add, though enforcing the read/write restrictions might require a change in libuv). The rest of the code is self-contained and self-explanatory. Compiled, tested, and run (on a cluster no less =).