caller.py assumes a lot on the environment

[dhrp]

rtlite.app.sip.caller has a couple of issues

• it assumes socket(gethostbyname(socket.gethostname)) returns an IP, but not all machines have their /etc/hosts file setup correctly for this to work. caller.py currently throws something like

socket.gaierror: [Errno 8] nodename nor servname provided, or not known

• it assumes os.getlogin() works, but this doesn't work when you're not logged in as a user (e.g. when you're running in a Docker container).

I did a quick looks and it seems to be hard to make these things work in a true cross-platform way, so I suggest catching the errors, outputting a helpful message and quitting. And then adding a flag for setting hostname and user.