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/wasm-src/tcp_client.c

@@ -0,0 +1,62 @@
+/*
+ * Copyright (C) 2019 Intel Corporation.  All rights reserved.
+ * SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+ */
+
+#include <arpa/inet.h>
+#include <netinet/in.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <unistd.h>
+#ifdef __wasi__
+#include <wasi_socket_ext.h>
+#endif
+
+int
+main(int argc, char *argv[])
+{
+    int socket_fd, ret, total_size = 0;
+    char buffer[1024] = { 0 };
+    struct sockaddr_in server_address = { 0 };
+
+    printf("[Client] Create socket\n");
+    socket_fd = socket(AF_INET, SOCK_STREAM, 0);
+    if (socket_fd == -1) {
+        perror("Create socket failed");
+        return EXIT_FAILURE;
+    }
+
+    /* 127.0.0.1:1234 */
+    server_address.sin_family = AF_INET;
+    server_address.sin_port = htons(1234);
+    server_address.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
+
+    printf("[Client] Connect socket\n");
+    if (connect(socket_fd, (struct sockaddr *)&server_address,
+                sizeof(server_address))
+        == -1) {
+        perror("Connect failed");
+        close(socket_fd);
+        return EXIT_FAILURE;
+    }
+
+    printf("[Client] Client receive\n");
+    while (1) {
+        ret = recv(socket_fd, buffer + total_size, sizeof(buffer) - total_size,
+                   0);
+        if (ret <= 0)
+            break;
+        total_size += ret;
+    }
+
+    printf("[Client] %d bytes received:\n", total_size);
+    if (total_size > 0) {
+        printf("Buffer recieved:\n%s\n", buffer);
+    }
+
+    close(socket_fd);
+    printf("[Client] BYE \n");
+    return EXIT_SUCCESS;
+}