Add strict-oidentd mode

Add strict-oidentd mode

This mode is disabled by default.  When specified, this restricts
authoratative ident responses to be the authuser account name.

This is a cut-down version of,
https://github.com/quassel/quassel/pull/164 which was originally
authored by @esainane.  @AlD requested that the database schema
change be removed, but the request fell dormant and was eventually
closed.  This version removes the schema change as requested,
always using the user's core username as the ident when enabled.

Co-authored-by: Sai Nane <esainane+github@gmail.com>
Co-authored-by: Michael Marley <michael@michaelmarley.com>