syscall: plan9 environment variable handling

[bradfitz]

While adding syscall.Unsetenv, I notice that plan9's environment handling in pkg syscall
seems inconsistent.

It sometimes does copyenv, but sometimes not. And things like Getenv can lazily populate
it. It seems like there are possible bugs with different orderings, but I didn't look
too hard.

Talking to Russ, he points out that plan9 processes can share environments, so keeping a
cached copy of anything in syscall is probably wrong.

I will do the bare minimum to keep plan9 compiling, but somebody should probably own
cleaning this up.

[bradfitz]

Comment 1:

Sorry, I did end up breaking plan9 with https://golang.org/cl/82040044
I think the reason is that Getenv and Setenv update the cached environment but don't
update the envi map I added.
I would try to fix this, but I don't have a way to test and in my opinion this code
needs an overhaul, so I'll leave it to people who can test.
I would be happy to review, though.

[0intro]

Comment 2:

Thanks. We have found the little issue. Nick will send a CL.

[gopherbot]

Comment 3:

CL https://golang.org/cl/149300046 mentions this issue.

[0intro]

Comment 4:

Owner changed to @0intro.

[gopherbot]

Comment 5:

CL https://golang.org/cl/158970045 mentions this issue.

[0intro]

Comment 6:

This issue was closed by revision 70896a7.

Status changed to Fixed.