net: implement closeRead and closeWrite on Plan 9

[0intro]

We should implement closeRead and closeWrite on Plan 9, so the following tests could be enabled:

• TestCloseRead
• TestCloseWrite
• TestClientWriteShutdown (net/http package, see net/http: failing tests on Plan 9 #7237)

[gopherbot]

CL https://golang.org/cl/35178 mentions this issue.

[gopherbot]

CL https://golang.org/cl/35179 mentions this issue.

[bradfitz]

I'm marking this as Unplanned but David, feel free to implement whenever you want, regardless of release cycle, as long as it only touches plan9 files or removes t.Skip calls.