sockstat: add -i to display inp_gencnt

The inp_gencnt will be used to identify a TCP endpoint by an upcoming
command line tool to set TCP socket options.

Reviewed by: rscheff
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D34137

(cherry picked from commit 5f64777a4fe38acb412762fa92da583a1018e609)