sockstat: show path state column only when useful

Showing the path state column is only useful, if there is at least one
SCTP endpoint shown, which is not in the state CLOSED or LISTEN.
Don't show it when it is not useful.

Reviewed by: rrs
MFC after: 3 days
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D52986