[Web] Implement dummy IPWeb instead of IPUnix

Note: This commit ties the IPUnix to the UNIX_SOCKET_UNAVAILABLE define,
disabling it when set. It is maybe not semantically correct (getifaddrs)
is not part of the "socket" API, but it's reasonable to expect that a
platform not supporting Unix-style sockets, would also not support other
Unix network functions.

platform/web/SCsub

@@ -26,6 +26,7 @@ web_files = [
     "http_client_web.cpp",
     "javascript_bridge_singleton.cpp",
     "web_main.cpp",
+    "ip_web.cpp",
     "os_web.cpp",
 ]