ctld: Refactor auth_group to support NVMe

Split out a private method to parse a listen address and optional
mask. This will avoid having to duplicate that code for NVMe
host addresses.

Rename the ag_names and ag_portals members to include "initiator"
to indicate they are iSCSI-specific.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D51728