cmd/vet: move the tests into separate files
Then mark them with a build tag so they're not compiled into the binary.
They are called test_*.go rather than *_test.go because they are not
for go test. Use make test to test the command.

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/7377052
net: Implement FileListener, FileConn, and File methods for Plan 9

Functions for representing network connections as files
and vice versa, on Plan 9.

Representing network connections as files is not so
straight-forward, because a network connection on Plan 9
is represented by a host of files rather than a single
file descriptor (as is the case on UNIX). We use the
type system to distinguish between listeners and
connections, returning the control file in the former
case and the data file in the latter case.

R=rsc, rminnich, ality, akumar, bradfitz
CC=golang-dev
https://codereview.appspot.com/7235068

Committer: Ron Minnich <rminnich@gmail.com>
cmd/vet: silence error from type checker unless verbose is set.
Also restores the checking of _test.go files, which disappeared
as a result of the package-at-a-time change.
Fixes issue 4895.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/7399051