Support 'senders' and 'received-auth' in 'listen on socket'

[Wolf480pl]

A proof-of-concept implementation of #709

• make the lookup process offer a uid to userinfo lookup by means of IMSG_SMTP_LOOKUP_UID_USERINFO
  (I wasn't sure if this should go through a table or not, so chose the simpler option for now)
• add an optional username argument to smtp_session
• use the uid received by smtp process in IMSG_CTL_SMTP_SESSION
  to find the user's username, and pass it to the smtp_session
• respect the F_RECEIVEDAUTH flag whenever a user is authenticated,
  even if the connection is not SF_SECURE
• allow using received-auth and senders in listen on socket in the config

[poolpOrg]

The feature request still exists but the PR was not acceptable back then and no longer applies, I'll close it and see if I can implement that differently.

For a back story, a lengthy discussion between @ericfaurot and I happened with regard to socket listeners, how they were implemented and how they should be implemented, a bit of the discussion provides insight on why your PR required special handling with regard to a network listener.

I'll tackle your feature request when we refactor a bit "listen on socket"