Allow to bind http port with loopback address.
This restrict access to http server only from localhost.
One could either specify 'loopback' option in constructor:
  new THttpServer("http:8080?loopback")
or in clear text specify IP address to which http socket should be bind:
  new THttpServer("http:127.0.0.1?loopback")
If host has several network interfaces, one could select one from them

Signed-off-by: Bertrand Bellenot <bertrand.bellenot@cern.ch>