Clean up the hostname look-up stuff:

- getnameinfo() was being called with NI_NUMERICHOST in the mail server and
  web server (but nowhere else)
- use a singly-defined macro (STR_NO_HOSTNAME) for the "<no name>" string
  rather than copying it about
- the webserver apparently assumes that session->host_name will always have
  *some* unique value (e.g. the IP address when no hostname is available) - so
  account for that special need <grumble>