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 <rminnich@gmail.com>
runtime: Plan 9, 32-bit: fix build by updating call to open()

With the global redefinition of runtime?open by CL 7543043,
we need to provide a third argument and remove the cast
to the string.

Fixes build on 386 version of Plan 9.

R=khr, rsc, rminnich, ality
CC=golang-dev
https://codereview.appspot.com/7644047

Committer: Ron Minnich <rminnich@gmail.com>