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

doc/socket_api.md

@@ -0,0 +1,66 @@
+# How to use Berkeley/Posix Socket APIs in WebAssembly
+
+**_Berkeley sockets_** usually means an API for Internet sockets and Unix domain
+sockets. A socket is an abstract representation of the local endpoint of a
+network communication path.
+
+Currently, WAMR supports a limit set of all well-known functions:
+`accept()`, `bind()`, `connect()`, `listen()`, `recv()`, `send()`, `shutdown()`
+and `socket()`. Users can call those functions in WebAssembly code directly.
+Those WebAssembly socket calls will be dispatched to the imported
+functions and eventually will be implemented by host socket APIs.
+
+This document introduces a way to support _Berkeley/Posix Socket APIs_ in
+WebAssembly code.
+
+## Patch the native code
+
+The first step is to include a header file of the WAMR socket extension in the
+native source code.
+
+```c
+#ifdef __wasi__
+#include <wasi_socket_ext.h>
+#endif
+```
+
+`__wasi__` is a Marco defined by WASI. The host compiler will not enable it.
+
+## CMake files
+
+It is recommended that the project should use CMake as its build system. Use
+[_wasi-sdk_](https://github.com/WebAssembly/wasi-sdk)
+as a toolchain to compile C/C++ to WebAssembly
+
+```bash
+$ cmake -DWASI_SDK_PREFIX=${WASI_SDK_DIR}
+      -DCMAKE_TOOLCHAIN_FILE=${WASI_TOOLCHAIN_FILE}
+      -DCMAKE_SYSROOT=${WASI_SYS_ROOT}
+      ..
+```
+
+In the *CMakeLists.txt*, include an extension of socket support and link with it.
+
+```cmake
+include(${CMAKE_CURRENT_SOURCE_DIR}/../../../core/iwasm/libraries/lib-socket/lib_socket_wasi.cmake)
+add_executable(socket_example tcp_server.c)
+target_link_libraries(socket_example socket_wasi_ext)
+```
+
+Now, the native code with socket APIs is ready for compilation.
+
+## Run with iwasm
+
+If having the _.wasm_, the last step is to run it with _iwasm_.
+
+The _iwasm_ should be compiled with `WAMR_BUILD_LIBC_WASI=1`. By default, it is
+enabled.
+
+_iwasm_ accepts address ranges via an option, `--addr-pool`, to implement
+the capability control. All IP address the WebAssebmly application may need to `bind()` or `connect()` should be announced first. Every IP address should be in CIRD notation.
+
+```bash
+$ iwasm --addr-pool=1.2.3.4/15,2.3.4.6/16 socket_example.wasm
+```
+
+Refer to [socket api sample](../samples/socket-api) for more details.