fixed overflow errors

% go test -run TestEnmarshalVarbind
./marshal_test.go:263: constant 4294967295 overflows int
./verax_test.go:159: constant 4294967295 overflows int
FAIL    _/home/sonia/go/bpl-poller/src/github.com/soniah/gosnmp [build
failed]
added port_on_outgoing1()
WIP - enmarshal - decode of set