os/exec: fix number of expected file descriptors on Plan 9

Since CL 104570043 and 112720043, we are using the
nsec system call instead of /dev/bintime on Plan 9.

LGTM=rsc
R=rsc
CC=aram, golang-codereviews
https://codereview.appspot.com/155590043
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