bsnmpd: merge send and send2 methods for transports

[glebius]

Summary

Shall be no functional change for normal SNMP sends, e.g. replies and
traps.

NB: for the inet_send() called from snmp_send_port() we now actually use
supplied address, but we could do a better job at finding matching socket.
However, it could be that snmp_send_port() is not used by any external
module, and in that case it is easier to just deprecate it as poorly
fitting design of the server. This will be checked with exp-run.

[harti]

I think snmp_send_port() was use by the ILMI daemon for ATM to proxy the ILMI tree (which was visible only between the two ILMI instances over the ATM) link into the regular tree visible via UDP. Since ILMI is long dead (R.I.P.) there should be no problem fixing these things.