syscall: fix StartProcess in Plan 9
syscall: fix StartProcess in Plan 9

This makes os_test.TestStartProcess test from os package pass.

R=paulzhol, r2, r
CC=golang-dev
http://codereview.appspot.com/4385052

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

tv42/old-go,ssrl/go,dsriram/golang,gnanderson/go,tv42/old-go,dsriram/golang,ssrl/go,tv42/old-go,pombredanne/go-deleteme,ssrl/go,ssrl/go,apparentlymart/golang,ssrl/go,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,apparentlymart/golang,gnanderson/go,dsriram/golang,apparentlymart/golang,ssrl/go,gnanderson/go,tv42/old-go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,dsriram/golang,dsriram/golang,apparentlymart/golang,dsriram/golang,ssrl/go,tv42/old-go