bsnmpd: deprecate legacy UDP transport trans_udp.c

Since 04d1781439aa we have a more generic trans_inet.c, that supports IPv6
and can be extended further. The default config file we ship was switched
to the new transport since 12.1-RELEASE, so for most people deprecation
would go unnoticed. Others [who never run etcupdate(8)] would need to
merge a couple lines into their snmpd.config. This will be reflected in
RELNOTES file.

Reviewed by: harti
Differential Revision: https://reviews.freebsd.org/D51359