ddb: optionally print inp when printing tcpcb

Add /i option to the ddb commands show tcpcb and show all tcpcbs,
which enables the printing of the t_inpcb.

Reviewed by: markj
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D53497

(cherry picked from commit 9aa5a79e2af9a6a8930bbe8a7a024df2cf44e433)