ctld: Only check physical port linking in a single configuration context

[jhb]

Summary

Commit 969876fcee57 moved struct pport from being per-configuration to
being a "global" object shared across multiple configurations. As a
result, the check for duplicate ports actually spanned across
configurations, such that reloading a configuration would now think
that existing physical ports were alreay linked.

The linking field in pport added in the C++-ification (commit
6acc7afa34aa) faithfully replicated this bug (albeit simpler as I had
noticed that the TAILQ links weren't used after the earlier commit).

To restore the desired behavior, remove the linking field from struct
pport entirely and use a local unordered_map in conf::add_pports which
tracks if a given pport is claimed by more than one target.

PR: 293076
Reported by: Ken J. Thomson <[e-mail]>
Fixes: 969876fcee57 ("ctld: parse config file independently of getting kernel info")
Sponsored by: Chelsio Communications

[jhb]

I was able to effectively test this today using pre-existing ioctl ports.