syscall: Plan 9: keep a consistent environment array

Map order is non-deterministic. Introduce a new
environment string array that tracks the env map.
This allows us to produce identical results for
Environ() upon successive calls, as expected by the
TestConsistentEnviron test in package os.

R=rsc, ality, rminnich, bradfitz, r
CC=golang-dev
https://codereview.appspot.com/7411047

Committer: Ron Minnich <rminnich@gmail.com>