Add "iscsictl -e". Among other things, it makes it possible to perform
discovery without attaching to the targets ("iscsictl -Ad ... -e off"),
and then attach to selected ones ("iscsictl -Mi ... -e on").

PR: 204129
MFC after: 1 month
Relnotes: yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D6633