runtime: handle non-nil-terminated environment strings on Plan 9
runtime: handle non-nil-terminated environment strings on Plan 9

Russ Cox pointed out that environment strings are not
required to be nil-terminated on Plan 9.

LGTM=rsc
R=rsc
CC=golang-codereviews
https://codereview.appspot.com/159130044

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