Add IPv6 transport for bsnmp.

Add IPv6 transport for bsnmp.

This patch adds a new table begemotSnmpdTransInetTable that uses the
InetAddressType textual convention and can be used to create listening
ports for IPv4, IPv6, zoned IPv6 and based on DNS names. It also supports
future extension beyond UDP by adding a protocol identifier to the table
index. In order to support this gensnmptree had to be modified.

Submitted by:   harti
MFC after:      1 month
Relnotes:       yes
Differential Revision:  https://reviews.freebsd.org/D16654