ddb: improve show tcpcb

Print the name of the TCP function block and the name of the
congestion control algorithm. Furthermore, print some information
related to Black Box Logging.

Reviewed by: thj
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D50535

(cherry picked from commit 8d4f495df46567dcfcbec59794e9b9137b2f38fe)