Fix immediate crash when snmpd is bound to a specific IP address.

The code that sets up msghdr must first fully fill in the msghdr
itself, and only then use CMSG_xxx() macros.

Silence from: harti, one week

Fix the byte order of IPv4 address parsed from begemotSnmpdTransInetStatus
config option.

An address is already in network byte order, there is no need to do
htonl().

PR: 242056
MFC after: 1 week

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