ctladm: print port number with a succesful "port -c" command

Make "ctladm port -c" print the port number of the newly successful
port. This way it won't have to be guessed by a subsequent "ctladm
portlist" command. That means it's safe to use it concurrently with
other ctladm processes. In particular, this allows the tests to be run
in parallel.

MFC after: 2 weeks
Sponsored by: Axcient
Reviewed by: mav
Pull Request: https://github.com/freebsd/freebsd-src/pull/1279

(cherry picked from commit 591de7534fb3acb2e6eef94a1e5e92000d2cf83d)