tcp_info: Add and export more FreeBSD-specific fields

This change adds struct tcp_info fields corresponding to the following
struct tcpcb ones:

• snd_una
• snd_max
• rcv_numsacks
• rcv_adv
• dupacks

Note that while both tcp_fill_info() and fill_tcp_info_from_tcb() are
extended accordingly, no counterpart of rcv_numsacks is available in
the cxgbe(4) TOE PCB, though.

Sponsored by: NetApp, Inc. (originally)