net-mgmt/net-snmp: Fix the IPV6 MIB implementation

The implementations of var_tcp6() and var_udp6() on FreeBSD don't work
and fail to compile. var_tcp6() also makes uses of kmem access, which
I'm trying to disable in snmpd. Provide a patch to fix these problems.
This has been committed to the 5.9 branch upstream.

Readd mibII/ipv6 since it now compiles and works in my manual testing.
Also, simply remove the IPV6 option. It has no effect, as --enable-ipv6
is the default.

Approved by: zi
Sponsored by: Klara, Inc.
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D45030