ctld: Be more consistent for auth parameters in the UCL config

The auth-group context required an array of entries for "chap",
"chap-mutual", "initiator-name", and "initiator-portal" whereas the
target context required exactly one entry (and only permitted a single
entry).

Allow either a single entry or an array of entries for these keywords
in both the auth-group and target contexts.

Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D48935