net: Plan 9: add lookupProtocol
net: Plan 9: add lookupProtocol

Needed by TestResolveIPAddr. This makes us pass tests
again.

R=rsc, rminnich, ality, bradfitz
CC=golang-dev
https://codereview.appspot.com/7737043

Committer: Ron Minnich <f569d0af9a3481c4007b95c7180d6c41009f2513@gmail.com>

webfd/go-zh,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,d0f/go-zh,sanjosh/sanjos100-tipc,webfd/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,bryanxu/go-zh,webfd/go-zh,bryanxu/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,bryanxu/go-zh,d0f/go-zh,d0f/go-zh,scirelli/scirelli-go,rdp/rogerpack2005-golang,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,rdp/rogerpack2005-golang,scirelli/scirelli-go,scirelli/scirelli-go,scirelli/scirelli-go,mhennings/marcohennings-go,webfd/go-zh,webfd/go-zh,d0f/go-zh,d0f/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,d0f/go-zh,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,mhennings/marcohennings-go,d0f/go-zh,rdp/rogerpack2005-golang,mhennings/marcohennings-go,webfd/go-zh,sanjosh/sanjos100-tipc,scirelli/scirelli-go,mhennings/marcohennings-go,scirelli/scirelli-go,scirelli/scirelli-go,webfd/go-zh,rdp/rogerpack2005-golang