net: fix LookupSRV ordering on plan 9

lookup_plan9.go's lookupSRV is using the wrong order for srv results. order should be weight, priority, port, following the response from /net/dns:

  chi Aug  9 20:31:13 Rread tag 20 count 61 '_xmpp-client._tcp.offblast.org srv 5 0 5222 iota.offblast.org' 72

R=golang-dev, bradfitz
CC=ality, golang-dev, r, rsc
https://codereview.appspot.com/12708043

Committer: Brad Fitzpatrick <bradfitz@golang.org>