Implement part of Berkeley Socket API for libc-wasi (#1036)

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

samples/socket-api/README.md

@@ -0,0 +1,57 @@
+"socket-api" sample introduction
+================================
+
+This sample demonstrates how to use WAMR socket-api to develop wasm network applications.
+Two wasm applications are provided: tcp-server and tcp-client, and this sample demonstrates
+how they communicate with each other.
+
+## Preparation
+
+Please install WASI SDK, download the [wasi-sdk release](https://github.com/CraneStation/wasi-sdk/releases) and extract the archive to default path `/opt/wasi-sdk`.
+And install wabt, download the [wabt release](https://github.com/WebAssembly/wabt/releases) and extract the archive to default path `/opt/wabt`
+
+## Build the sample
+
+```bash
+mkdir build
+cd build
+cmake ..
+make
+```
+
+The file `tcp_server.wasm`, `tcp_client.wasm` and `iwasm` will be created.
+And also file `tcp_server` and `tcp_client` of native version are created.
+
+Note that iwasm is built with libc-wasi and lib-pthread enabled.
+
+## Run workload
+
+Start the tcp server, which opens port 1234 and waits for clients to connect.
+```bash
+cd build
+./iwasm --addr-pool=0.0.0.0/15 tcp_server.wasm
+```
+
+Start the tcp client, which connects the server and receives message.
+```bash
+cd build
+./iwasm --addr-pool=127.0.0.1/15 tcp_client.wasm
+```
+
+The output of client is like:
+```bash
+[Client] Create socket
+[Client] Connect socket
+[Client] Client receive
+[Client] 115 bytes received:
+Buffer recieved:
+Say Hi from the Server
+Say Hi from the Server
+Say Hi from the Server
+Say Hi from the Server
+Say Hi from the Server
+
+[Client] BYE
+```
+
+Refer to [socket api document](../../doc/socket_api.md) for more details.