Implement Berkeley Socket API for Intel SGX
- bring Berkeley socket API in Intel SGX enclaves,
- adapt the documentation of the socket API to mention Intel SGX enclaves,
- adapt _iwasm_ in the mini-product _linux-sgx_ to support the same option as the one for _linux_,
- tested on the socket sample as provided by WAMR (the TCP client/server).
Refer to [Networking API design](https://github.com/WebAssembly/WASI/issues/370)
and [feat(socket): berkeley socket API v2](https://github.com/WebAssembly/WASI/pull/459):
- Support the socket API of synchronous mode, including `socket/bind/listen/accept/send/recv/close/shutdown`,
the asynchronous mode isn't supported yet.
- Support adding `--addr-pool=<pool1,pool2,..>` argument for command line to identify the valid ip address range
- Add socket-api sample and update the document
