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_server.c

@@ -0,0 +1,118 @@
+/*
+ * 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 <pthread.h>
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <stdint.h>
+#include <string.h>
+#include <sys/socket.h>
+#include <unistd.h>
+#ifdef __wasi__
+#include <wasi_socket_ext.h>
+#endif
+
+#define WORKER_NUM 5
+
+void *
+run(void *arg)
+{
+    const char *message = "Say Hi from the Server\n";
+    int new_socket = *(int *)arg;
+    int i;
+
+    printf("[Server] Communicate with the new connection #%u @ %p ..\n",
+           new_socket, (void *)(uintptr_t)pthread_self());
+
+    for (i = 0; i < 5; i++) {
+        if (send(new_socket, message, strlen(message), 0) < 0) {
+            perror("Send failed");
+            break;
+        }
+    }
+
+    printf("[Server] Shuting down the new connection #%u ..\n", new_socket);
+    shutdown(new_socket, SHUT_RDWR);
+
+    return NULL;
+}
+
+int
+main(int argc, char *argv[])
+{
+    int socket_fd = -1, addrlen = 0;
+    struct sockaddr_in addr = { 0 };
+    unsigned connections = 0;
+    pthread_t workers[WORKER_NUM] = { 0 };
+    int client_sock_fds[WORKER_NUM] = { 0 };
+
+    printf("[Server] Create socket\n");
+    socket_fd = socket(AF_INET, SOCK_STREAM, 0);
+    if (socket_fd < 0) {
+        perror("Create socket failed");
+        goto fail;
+    }
+
+    /* 0.0.0.0:1234 */
+    addr.sin_family = AF_INET;
+    addr.sin_port = htons(1234);
+    addr.sin_addr.s_addr = htonl(INADDR_ANY);
+
+    printf("[Server] Bind socket\n");
+    addrlen = sizeof(addr);
+    if (bind(socket_fd, (struct sockaddr *)&addr, addrlen) < 0) {
+        perror("Bind failed");
+        goto fail;
+    }
+
+    printf("[Server] Listening on socket\n");
+    if (listen(socket_fd, 3) < 0) {
+        perror("Listen failed");
+        goto fail;
+    }
+
+    printf("[Server] Wait for clients to connect ..\n");
+    while (connections < WORKER_NUM) {
+        client_sock_fds[connections] =
+            accept(socket_fd, (struct sockaddr *)&addr, (socklen_t *)&addrlen);
+        if (client_sock_fds[connections] < 0) {
+            perror("Accept failed");
+            break;
+        }
+
+        printf("[Server] Client connected\n");
+        if (pthread_create(&workers[connections], NULL, run,
+                           &client_sock_fds[connections])) {
+            perror("Create a worker thread failed");
+            shutdown(client_sock_fds[connections], SHUT_RDWR);
+            break;
+        }
+
+        connections++;
+    }
+
+    if (connections == WORKER_NUM) {
+        printf("[Server] Achieve maximum amount of connections\n");
+    }
+
+    for (int i = 0; i < WORKER_NUM; i++) {
+        pthread_join(workers[i], NULL);
+    }
+
+    printf("[Server] Shuting down ..\n");
+    shutdown(socket_fd, SHUT_RDWR);
+    sleep(3);
+    printf("[Server] BYE \n");
+    return EXIT_SUCCESS;
+
+fail:
+    printf("[Server] Shuting down ..\n");
+    if (socket_fd >= 0)
+        close(socket_fd);
+    sleep(3);
+    return EXIT_FAILURE;
+}