ctld: Factor out code to setup a listening socket into a separate function

While here:

• Reuse the address family from the addrinfo structure instead of calling getsockname(2) on the socket.

• Remove a bogus comment about the PCP option.

Reviewed by: mav, asomers
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D48596