Log Message:
net-snmp: NetBSD >= 9.99.104 build fixes (part one)

Address fallout from struct inpcb changes in NetBSD 9.99.104, as
impacts ipv6.c, specifically. Note this is compile-tested only (on
9.99.108 and 9.3_STABLE), I don't use this package (nor any of its
dependants) and haven't tested the functionality.

(At least tcpTable.c and udpTable.c also need adjustments. I don't have
the time or patience to deal with them at the moment, I'm so checking
in what I've adjusted so far.)

(The upstream code in ipv6.c uses two inconsistent approaches to handle
differences in NetBSD versions. I opted for the more explicit code
blocks everywhere, to avoid confusion. I didn't bother using the new
macros added in 9.99.104 to fetch specific struct members, as they
don't cover much of what's needed here anyway, and would arguably just
make things look even more ugly.)