Implement NetworkManager

For now there are two methods:

- `NETWORK:IsUrlAllowed()`: Check whether access to a certain URL is allowed.
- `NETWORK:HttpRequest()`: Perform an HTTP request.

By default access to the network is disabled for all target hosts. It
can be enabled by setting `HttpEnable=1` in the preferences. Individual
hosts have to be added to `HttpAllowHosts` as a comma separated list to
allow access.

See included docs for more details on usage.

extern/IXWebSocket-11.3.2/ixwebsocket/IXNetSystem.h

@@ -0,0 +1,86 @@
+/*
+ *  IXNetSystem.h
+ *  Author: Benjamin Sergeant
+ *  Copyright (c) 2019 Machine Zone. All rights reserved.
+ */
+
+#pragma once
+
+#ifdef _WIN32
+
+#ifndef WIN32_LEAN_AND_MEAN
+#define WIN32_LEAN_AND_MEAN
+#endif
+
+#include <WS2tcpip.h>
+#include <WinSock2.h>
+#include <basetsd.h>
+#include <io.h>
+#include <ws2def.h>
+
+#undef EWOULDBLOCK
+#undef EAGAIN
+#undef EINPROGRESS
+#undef EBADF
+#undef EINVAL
+
+// map to WSA error codes
+#define EWOULDBLOCK WSAEWOULDBLOCK
+#define EAGAIN WSATRY_AGAIN
+#define EINPROGRESS WSAEINPROGRESS
+#define EBADF WSAEBADF
+#define EINVAL WSAEINVAL
+
+// Define our own poll on Windows, as a wrapper on top of select
+typedef unsigned long int nfds_t;
+
+// pollfd is not defined by some versions of mingw64 since _WIN32_WINNT is too low
+#if _WIN32_WINNT < 0x0600
+struct pollfd
+{
+    int fd;        /* file descriptor */
+    short events;  /* requested events */
+    short revents; /* returned events */
+};
+
+#define POLLIN 0x001   /* There is data to read.  */
+#define POLLOUT 0x004  /* Writing now will not block.  */
+#define POLLERR 0x008  /* Error condition.  */
+#define POLLHUP 0x010  /* Hung up.  */
+#define POLLNVAL 0x020 /* Invalid polling request.  */
+#endif
+
+#else
+#include <arpa/inet.h>
+#include <errno.h>
+#include <fcntl.h>
+#include <netdb.h>
+#include <netinet/in.h>
+#include <netinet/ip.h>
+#include <netinet/tcp.h>
+#include <poll.h>
+#include <sys/select.h>
+#include <sys/socket.h>
+#include <sys/stat.h>
+#include <sys/time.h>
+#include <unistd.h>
+#endif
+
+namespace ix
+{
+#ifdef _WIN32
+    typedef SOCKET socket_t;
+#else
+    typedef int socket_t;
+#endif
+
+    bool initNetSystem();
+    bool uninitNetSystem();
+
+    int poll(struct pollfd* fds, nfds_t nfds, int timeout);
+
+    const char* inet_ntop(int af, const void* src, char* dst, socklen_t size);
+    int inet_pton(int af, const char* src, void* dst);
+
+    unsigned short network_to_host_short(unsigned short value);
+} // namespace ix