ar: deprecate -T option

Other ar implementations (GNU, LLVM) use -T to mean thin archive
rather than use only the first fifteen characters of the archive member
name. We support both -T and -f for this, with -f documented as an
alias of -T.

An exp-run showed that the ports invoking ar -T expect thin archives,
not truncated names. Switch -f to be the documented flag for this
behaviour, and emit a warning when -T is used.

The warning will be changed to an error in the future (in main), once
ports no longer use -T.

PR: 260523 [exp-run]
MFC after: 1 week
Sponsored by: The FreeBSD Foundation