Add username in nnrpd's process title

[Julien-Elie]

The username (when set) could be added at the beginning of ps output:

  nnrpd: accepting connections
  nnrpd: julien 176-143-2-105.abo.bbox.fr GROUP
  nnrpd: <all> 1.2.3.4 LIST

For instance in nnrpd/nnrpd.c:

setproctitle("%s %s %s", PERMuser[0] != '\0' ? PERMuser : "-", Client.host, av[0]);

The goal is to have a way for possible auth filters to find out how many active connections a given user has. See #23.

For privacy concerns, this feature should be configurable with a readers.conf option (like addprocesstitleuser which would enable that behaviour when set to true in an access group).