examples/http_server.py: Introduce main() function.

Allows to re-run code if it was imported as a module (e.g., on bare-metal
ports).

examples/network/http_server.py

@@ -10,6 +10,7 @@ HTTP/1.0 200 OK
 Hello #%d from MicroPython!
 """
 
+def main(use_stream=False):
     s = socket.socket()
 
     ai = socket.getaddrinfo("127.0.0.1", 8080)
@@ -29,7 +30,7 @@ while True:
         print("Client address:", client_addr)
         print("Client socket:", client_s)
         print("Request:")
-    if 0:
+        if use_stream:
             # MicroPython socket objects support stream (aka file) interface
             # directly.
             print(client_s.read(4096))
@@ -40,3 +41,6 @@ while True:
         client_s.close()
         counter += 1
         print()
+
+
+main()