systat: clean up code assuming network classes

Similar to netstat, clean up code that uses inet_lnaof() to check for
binding to "host 0" (lowest host on network) as a "network" bind.
Such things don't happen, and current networks are seldom if ever
found in /etc/networks.

MFC after: 1 month
Reviewers: tuexen
Differential Revision: https://reviews.freebsd.org/D32720