ctladm: deprecate the undocumented "port -l" option

It was mostly removed from the man page in
9c887a4f86f5fd4f51c23443dc8435e52783a782, but left in the command.
Fully remove it from the man page, and warn if anybody uses it. Remove
it entirely for FreeBSD 16.

PR: 279546
MFC after: 2 weeks
Reviewed by: mav
Sponsored by: Axcient
Differential Revision: https://reviews.freebsd.org/D45522