ddb: add optional printing of BBLog entries

Add a /b option to show tcpcb and show all tcpcbs to print BBLog
entries. Right now this supports the entries generated by the
FreeBSD default TCP stack. It should help in debugging issues
reported by syzkaller.
The syntax for printing sent and received packets is similar to the
one used by packetdrill, since the output of ddb will be used to
create packetdrill scripts for debugging.

Reviewed by: thj
Tested by: thj
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D50629