syscall: fix Setenv for plan 9
syscall: fix Setenv for plan 9

envi needs to be updated during Setenv so the key can be correctly deleted later with Unsetenv.

Update issue 8849.

LGTM=0intro
R=bradfitz, 0intro
CC=golang-codereviews
https://codereview.appspot.com/149300046

Committer: David du Colombier <ec8ae1e6620d4bab5e0e7aa2a6ae41aad63e3708@gmail.com>

tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go,tv42/old-go