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ctl: Make max_luns and max_ports tunable variables instead of hardcoded

Description

ctl: Make max_luns and max_ports tunable variables instead of hardcoded
defines.

Reviewed by: trasz (earlier version), bapt (earlier version), bcr (manpages)
MFC after: 2 Weeks
Sponsored by: Gandi.net
Differential Revision: https://reviews.freebsd.org/D12836